It doesn't matter what gear I'm in, as soon as I hit 2000rpm it shakes a little, then by 2500 its gone. I'm not sure when the front driveshaft was replaced. Just did plugs and wires. It feels like a drivetrain issue, but I'm not sure. Any clues?

1996 D1 5spd

 

Take serpantine belt off and repeat test to eliminate something that turns at engine speed. Seems like if it does it in 1st or 5th at same rpm, would not be drive train (other than tranny). But also look at rear shaft rubber coupling.

 

What is the rear shaft rubber coupling, the rotoflex thing?

 

yep that's what I meant.

 

Thanks for the quick reply.

Mine looks somewhere between the two of those... certainly not nearly as torn up as the one in the picture though. Just pretty dirty and a little "cracky." I tried to get it to shake while sitting still in neutral, and couldn't feel it shaking at all. What moves when in gear at 2k but doesn't when in neutral?

Thanks for all the help, wish I understood a little more about how vehichles work.

 

If you put the front shifter (CDL) in "N" then you can engage engine clutch, transmission will operate, but transfer case won't send any of this to drive shafts. Transfer case can go bad ($$$), drive shafts can have bad U joints or cracked yokes, and so on. If U joints can't be greased it is most likely time to replace them. If they can be greased, this is a regular maintenance item. The of course there are bad tires, bent wheels, wheel bearings, etc. Once the problem is found to be when under way, anything that rotates can be suspect from transfer case to tires.

But if it does it at 2000 in 3rd, and 2000 in 4th and 2000 in 5th, seems like that takes out the drive train because all of it is moving at different speeds when rpm is at that level. So perhaps motor or some other mount?

My point is that if it is RPM dependent in different gears, that would have to happen before the drive shafts.

 

Thanks a ton, it actually feels like an engine mount may be failing as it bucks a little sometimes. I'll definitely check that out today. What you described is exactly correct, 2000rpm in every gear until actually about 2800 rpm, then it goes away.

 

These trucks are old and getting to the point where the high mileage ones need just about every rubber bushing and wear component replaced. That's no small undertaking and it can also make vibration problems hard to diagnose.
Could be:
Engine mounts, transfercase mounts, transfercase front and/or rear output bearings, front and/or rear ujoints/rotoflex.
If your ujoints and rotoflex are all original you're probably due to replace them regardless if they're giving you a problem at this specific moment.

 

I'm not sure if the ujoints and rotoflex are original, I just bought the truck a few months ago. Its got 136k on it.

 

I'd drop the driveshafts and check the Ujoints for play in the wrong directions. Also inspect the RotoFlex. While the Driveshafts are out put it in Drive and see if vibration is still there at 2000 RPM's.