Vibes in 4wd

Ok first the spec's 87 with 6.5" lift 242 t-case and a 2 year old front ds.

Now the problem, when I put it in 4wd I get some prety bad vibes at about 45 mph. Now I don't understand how it could be the drive shaft because it spins all the time. I get the vibes during accleration and decelaration. As soon as I put it back in 2wd it's gone. What's the dillio? TIA. Neil

First off ... I'm by no means an expert at this so forgive me if I post any bad info ...

I drive a '94 with a 231 TC on a 4.5" lift and have the same vibration symptons you're posting about. I am working on correcting them now.

My problem, though, started with several issues. I was geeting vibration in both 2WD and 4HWD. At that time, I had a severly worn out axle hub on the passenger side that was giving me vibration. I replaced it and the axle U-Joint and got rid of a lot of vibes, but still had some vibration.

I removed the front drive shaft and found a U-Joint that, was not bad, but it did not rotate smoothly. I replaced it because of this and because it was a sealed joint. I checked the shaft for damage and DC joint and they seemed to be in very good shape. After doing this, however, It did not improve the vibes, but I do think I've isolated the problem with the front drive shaft. If all else fails, I'm going to pull it off and have it checked at a Drive Shaft Repair shop.

I did a "search" on this forum (which I find very helpful) and saw a post to check the transfer case fluid level. Well ... dumba$$ me let it get way too low. I brought it up to level and it made some (~) improvement.

I am now tweeking in the front pinion angle. I have a fixed LCA for a 3" lift (~16" eye to eye). I removed the shims and pulled the arm rearward which tipped the front pinion up to within' I'd guess 5-10 degrees of being lined up with the drive shaft. It removed all the vibes in 2WD and improved the vibes when in 4HWD, but not near enough. Since both the UCA's and LCA's on mine are fixed, I have ordered adjustable UCA's to work with this angle some more (I'll need them soon anyway if plans all work out). I'm sure my caster will not be right, but I'll address that after solving this vibration problem.

I currently have the pretty much the same symptons you have. Hardly any vibes in 2WD, but bad vibes in 4WD especially at around 45 mph. There is a "sweet spot" where they are not bad. Now, when I drive in 4HWD, I just keep it under 45mph.

I'm sorry I cannot be of much help right now, but keep at it and don't get discouraged. Patience is a virtue you have to develope especially when modifying your vehicle. I'll keep you posted on what I find ... and I'd appreciate it you'd post back if you solve yours.