weird vibration problem

[77'cj7]

Several months ago, I was driving down a gravel road, and started getting vibrations bad enough that i thought I was loosing a wheel. I was finally able to make the vibrations stop by shifting the t-case into 4 wheel. (I have manual hubs and a manual t-case) Couldn't drive over 30 or so in 2 wheel without bad vibrations starting up after that. Had the t-case flushed, and the problems went away.
Since that case has always had excessive chain noise, and the vibration thing scared me, I was able to procure a brand new 1354 manual on ebay, which I swapped out for it last week.
Driving on the interstate yesterday, the vibrations returned. Shifting into 4 wheel gets rid of them. Had the transfer case flushed today, no change.
Tried removing the counterweight from the t-case, only made it worse.

Any idea what could be causing this problem? The case isnt stock for the truck, however, it worked fine for a year or so. 1354E to 1354M swap. Truck also has explorer rear axle.

Thanks
Chris

 

[4x4RangerGuy]

Are your ujoints in good shape?

 

[77'cj7]

They feel tight and don't make any noise. I will probably have the driveshaft balanced next.

 

[4x4RangerGuy]

It could possibly be that your rear shaft is out of balance.

Also, check to make sure your ebrake isn't even the slightest bit applied. I've done this before when it was just slightly engaged, not enough to make the light go on, but enough to cause an issue.

 

[77'cj7]

Mkay, i'll check the e-brake. Doesn't make sense that shifting the t-case into 4 wheel would cause it to stop, but can't hurt to check.

 

[Capt Jay]

My vote is a bad U-joint on the rear drive shaft. It doesn't take much for it to start vibrating. Had a similar problem with my old Ranger. U-joint felt OK until I put it all in neutral and took the pressure off the shaft. The rear joint didn't have a needle bearing left in it. I'd check them again. Any little movement and you should replace it/them.