I bought .a 98 ram 1500 quad cab 4x4 a couple months ago.on test drive I could feel a vibration in seat and have been trying to troubleshoot it.i have removed driveshaft and was told by driveline specialist that u joints were good.i also rotated wheels and tires front to rear with no change.when I was initially trying to inspect u joints I noticed bad trans mount and replaced that.so now the problem continues..a shaking in seat and floor between 35 and about 55 mph.coasting, braking, putting trans in neutral do nothing to change it. There is fluid in the extension housing seal on the transfer case but I can't get movement there when pushing yoke. I would like to avoid throwing money at it and guessing if possible, but I don't know how to move forward with it.i was very surprised moving the tires didn't move the vibration to the steering wheel.there is also a noise kind of an oscillating wa wa wa noise that isn't real loud and could be tire noise but I don't think so. Suggestions?

 

If you lightly apply the brakes, does the vibration change? Is the T-case in some flavor of 4x4?

 

Braking or acceleration does nothing, other than to get out of the range it does it. In 2 hi.
It's really consistent and I've not found anything to make it change other than getting out of that mph range.my instinct says tire despite kind of ruling it out.

 

I would tend to agree with you. Wouldn't hurt to have 'em checked/balanced.... maybe you have more than one that is bad.

 

It doesn't make sense that I couldn't feel a bad front tire in the steering wheel before or after rotation though.

 

Where did you get the idea that problems of this nature need to make sense?

If it was a front end problem, yes, you would indeed expect to feel it thru the steering wheel. Given that it happens in a specific speed range, regardless of other circumstances though, gotta be something in the driveline that spins... So, maybe an out of balance driveshaft, bad or sticky u-joints, or possibly a bearing in the rear diff. (though I would expect to be able to HEAR that.) One or two piece rear driveshaft on yours?

 

One piece, steel driveshaft

 

An interesting experiment might be to pull the rear shaft, put it in 4x4, and run it up to speed. See if the shake is gone. Gotta do something with the rear output though, else it will puke fluid, and empty the t-case right quick.... Maybe run it on jack stands?

 

Exactly what I've been thinking and also run it on stands with the wheels removed and drums tightened down with a couple lug nuts.

 

That would likely work.