I just read a few dozen threads on the topic so I have a pretty good idea of the direction I'm heading with this but wanted to throw my story out to those here with more way knowledge than me on the issue.

New to me 2004 Disco II, has a vibration that starts around 38mph and comes and goes up through 75mph (haven't gone faster). Never completely goes away but gets noticeably smoother at times, IE 51mph seems to be a sweet spot were it's (usually) mostly imperceptible. First I swapped the wheels with a known balanced spare set I've run on my 2003 but there was no change. It doesn't feel like unsprung weight though, the frequency is much higher as it would be if it were ahead of the diff. The steering wheel vibrates a little but doesn't shake, kind of like you're running over rumble strips just not nearly that violent, and definitely is being transmitted through the chassis. Next I put in a new Lucky8 front driveshaft. The old one's cardan joint was not greaseable so that was on my list anyway, but no change. Then I put in a known good spare rear driveshaft and a new flex coupling but no change to the characteristics of the vibration.

Next step is to check all the fluids. When I bought the truck to PO texted me a few days later and said "the diff fluid was just changed to Royal Purple" without specifying which one but I will assume both. After a 15 minute mostly highway ride yesterday front diff felt slightly warmer than the rear, but there wasn't much temp to either. About a half hour later it occurred to me to check the trans and t/c too. Trans pan was warm, t/c was bordering on hot. I plan to remove the driveshafts separately and lock the CDL to isolate the diffs (wish I had thought of that yesterday when I was swapping them out...). My big fear is the output shaft bearing in the transfer case. I have a spare set of axles but my spare center diff is from an 03 so it's not locking. I also read that motor and trans mounts should be checked. The only other thing of note is that in 2015 at 122k miles the original front driveshaft blew up and flailed a hole in the trans so it was replaced with and 83k mile trans for $3,500, but there's no mention of the t/c being replaced with it. What is does say on the RO however is "Note: There is a slight vibration while driving". I'm wondering if it's been like this for the last 50k.

Maybe the old driveshaft beat the crap out of the t/c before it let go, or the diffs are spent from poor maintenance but that seems like a long shot, or maybe something as simple as engine/trans mounts (even longer shot). Open to any and all thoughts on the issue. Planning to dig into it over the next few days and will update with my findings.

 

well, back to driveshaft...remove the front and drive it...see what happens....

 

OK, front driveshaft removed and zero vibration at the way up to 80mph, which is as fast as I'll ever need to go in a Disco. I felt the output flange and there wasn't the slightest wobble to it that I could feel so I'm assuming front driveshaft is the culprit.

So what are the next steps to diagnosing the front end? I assume the fluid level is good since the PO went out of his way days after I bought it to inform me it was just changed to Royal Purple, however I will verify the level. The front driveshaft could certainly be out of balance, despite being brand new, however the vibration was the same before I installed it. Is it smarter to piece meal the front driveshaft or just swap in the full spare unit, calipers and all? It's a known good setup with nearly identical mileage, coincidentally enough.

 

sounds like you found the issue...driveshaft....new or old...can be out of balance....

if something was wrong with the front axles, you would have felt it....

seems that this is an issue for many of us from 40 mph on up. You could take it to a shop with multi plane balancer and see what happens....

I had an old version of a HD TW D-shaft which was smooth as glass until the center ball dried up. I bought a new TW HD D-shaft which was waaay out for my rig. and HD from another vendor another which was also waay out. Tried hose clamps, clocking the flanges for 12 positions.. nothing worked. Bought a cheapy light weight off ebay which has been very smooth...I don;t expect it to last very long...but it's greaseable.

in the meantime, I'm rebuilding my old TW with new center ball. Luckily, it uses basic D shaft spicer parts and a still available heavy duty ball flange....

 

Well, I don't disagree that the driveshaft is certainly on the list of suspects and I hope you're right. I guess the way to be sure is to take the known good driveshaft out of my 03 and slap it into this truck. The fact that the shop that put the replacement trans in noted the vibration, and it was vibrating the same way with the last driveshaft, I'm not washing my hands of this one yet.

 

OK, so I took this brand new Lucky8 driveshaft and put in in the 03 truck, which was previously vibration free, and the 03 remained vibration free. So the new driveshaft is not unbalanced and the cause of the vibration in the 04. Since the 04 is vibration free without a front driveshaft and the front driveshaft checks out ok then the problem is most likely to be the front axle somewhere. Back to my earlier question, do I swap the entire front axle from a parts truck or is it smarter to cannibalize individual parts? How like is it that the diff is the cause, vs the axles or hubs/rotors, etc etc. We're at the event horizon of my experience, I've never even worked on a live axle before so I'm not sure what's a common failure item, what's replaceable, etc. I suppose the output bearing on the transfer case could be second in line but I did not experience and play in the flange when I took the driveshaft off.

 

I'm going with the front output shaft of the TC... Not sure how much play you would feel by hand on that...

However what you said previously about the PO being very insistent on letting you know about the diff fluids could be his way of hinting around about something...

 

bearings for TC input shaft, intermediate shaft, output shafts are pre-loaded, so there should be no radial play you can feel with your hand.

 

Excellent points, both are very much on my radar. Since the cardan joint blew up in 2014 I have to assume it was driven for some amount of time with a good wobble, which would certainly stress the output shaft assembly. The Royal Purple thing also puzzled me a bit, seemed an odd thing to point out that way.

I guess it's easier to change a front axle assembly than it is to drop and disassemble the transfer case. Process of elimination will get me to the transfer case one way or the other, or not. Is there a more scientific way to check the output flange then shaking hands with it? I couldn't get it to wiggle at all. The only weird part to that theory is that it seems odd that the truck would have been driven in this condition for 50k miles.

 

well, yes, the bearing could have been damaged if the shaft tore itself in half.....if that's what happened? or it just 'went out'... but if flange tight, it's probably not the transfer case...

it this disco lifted?

I should clarify my post above...the TC output shaft with the diff on it has 4 bearings...2 taper and 2 ball. the taper bearings are preloaded. but there should still be no radial wiggle on the shaft...