EDIT: Mostly (completely?) solved. Check out my post #9 below- I removed the front prop shaft and found a U-joint in bad shape.

Today on my drive home from work I noticed a pronounced shake/shimmy that can be felt very clearly through the steering wheel. Here are the facts:
1) It seems to set in at about 35 mph and reduce around 65mph
2) It only occurs under load
3) No codes or unusual noises
4) It doesn’t correlate with engine speed, only road speed

It’s been colder/ more snowy here recently, if that makes a difference. The part that really confuses me is that it only happens under load. When I lift off the throttle is goes away, same when I shift in to neutral at speed. Any suggestions of where to start?

 

First thing l would check would be front driveshaft...vibration could be transmitting up through axle to steering components. Then if that checks out good, jack it up and shake every steering component you can get your hands on, tie-rod ends, drag-link, etc. Check ball joints by putting long bar under tire, while still on jackstands, and lift tire, have someone else watch ball-joints for up and down movement, should be none. How old are the shocks, tires and steering stabilizer shock? Also, you may have lost a wheel weight and balance of tire is compromised.

Normally, if you feel an issue in the steering wheel, it is front end related. If you feel issue in your seat, it's trans/rear driveline, tires.

 

Shocks, springs and front steering stabilizer all all brand new (<6mo old). My front driveshaft is aftermarket about 3 years old.

If I threw a wheel weight wouldn’t I feel that regardless of engine load?

 

I feel like I'm getting a bit of judder from the transmission when accelerating lightly from about 40-50ish. A bit more noticeable when cold. If I put the shifter in 3, then I don't feel it. The condition is barely noticeable under moderate or heavy throttle. Which might be the result of higher RPM/later shift going into 4th.

Maybe try locking out 4th and see if you still get it.

 

FWIW, I recently replaced my belt, tensioner and pulleys. Truck is much smoother and runs at 65 with minimal vibration after being a PITA for several years. Just an idea...

 

Also, check your spare tire carrier. I had a tire carrier that had failed at the studs and the spare tire shook the whole truck. I didn't realize what a difference it made until I removed the spare tire. New spare tire carrier is rock solid.

 

Thanks for the suggestions- I'll start my troubleshooting as always with the simple things, hopefully its just a thrown wheel weight. Naturally it has to happen when my driveway is frozen solid with ice but I should be able to sneak the front end in the garage to inspect.

 

soooo......unless your D shaft is super loose, you will not be able to feel it with your hands. Also, the front diff input bearing could be shot, and hard to tell that unless you pull the D shaft and play with the flange...even then, with the flange in place, hard to tell. usually, you have to pull the flange and feel the pinion shaft for any wobble. A bad pinion shaft bearing will definitely make your front end shaky at certain speeds...like 40 -60mph

Driveshaft joints could be worn and loose

and of course tires/wheels out of balance....

 

I was (finally) able to get back on the horse and continue troubleshooting this today. All of the steering and suspension components felt good after a cursory “shake test” so I removed the front drive shaft. There is a little bit of play in the rear U-joints, I can hear some minor clicking when I twist it back and forth which I assume shouldn’t happen. There isn’t any play or backlash in the front U-joint. I locked the center diff from underneath (yay for a ‘99!) and took it for a drive and there was no shaking. So at this point I guess I’m looking at a driveshaft rebuild? The driveshaft is a standard duty Lucky 8 replacement which is only 3 or 4 years old and I’ve been greasing it every to every other oil change. Is this one of those things where I just need to replace the faulty joint or is it worth it to replace them all? Is it difficult or should I just take it to a drive shaft shop?

 

i have had two of the driveshafts from Lucky8 fail on me. both within a year or so.

which reminds me, i need to call the shop to see if they ever completed the rebuild on both shafts.