Hey all,
Truck is a '99 4wd half ton with the 360. When I let off the gas to coast, there is a noticeable vibration. It gets worse it higher speeds, and goes away under 40mph. It only happens when I let off the gas. Throwing it in neutral (auto tranny) doesn't help at all.

I jacked the truck up and grabbed a hold of the rear driveshaft. There seemed to be some up and down play just behind the transfer case. I pulled the driveshaft out and grabbed a hold of the tail shaft to see if the tail shaft bearing was the problem. There didn't seem to be any abnormal play, it was solid.

So I replaced both U-joints in the driveshaft (they were originals, but seemed to be in good shape even after 150xxx miles). I re-installed the driveshaft and I still have the same problem!

My question is, is it possible that a bad transmission mount will cause a this problem? If I pull up and down on the tail shaft I can move the entire assembly about an inch, and I'm not a very big guy. I think the mount is shot, the rubber is cracked/soft and the inside metal part is pretty much non-existent. Could this be the issue?

 

Not sure about that tranny mount. When you feel the vibration do you hear any sounds from the same area?

 

Yes, there is a lot of noise. Sounds like engine braking almost.

 

Maybe related maybe not, but I had the same symptoms when my TS cracked.

1st post is my t-case.

https://dodgeforum.com/forum/2nd-gen...-it-pours.html

 

I'd guess something in the rear end like maybe failing carrier bearings. Especially since it seems independent of what the engine and tranny are doing.

 

More likely pinion bearings in th rear, the seal will start leaking soon if it isn't already. If not, it could be in the tailshaft of the transfercase(internal bearing). Not to say that the carriers could be going as well. You should be noticing a slight rumble on moderate acceleration from a coast also

 

I don't think it's in the transfercase, the tailshaft has no play in it at all. How do I check the carrier bearins and the pinion bearing? I have the diff cover off since I was going to change the fluid anyway. There weren't any chunks or grime in there at all, and the fluid still looked ok.

 

Barring getting in there and physically scrutinizing the components in question probably the best your going to do is find someone with a trained ear.