4 Wheel Drive/SUVs: Vibration, jeep dealer, dodge 2500

Question
Dear Sir,
I have a 1998 5.9 Grand Cherokee, I have already had the Hybrid shaft and frontend diff rebuilt (just for history sakes) I get a nasty vibration which seams to come from the rear end at about 50 mph and at other speeds as well, iot tends to happen when  I'm on the throttle but as soon as I let off it goes away, I have also just had the rear shaft U-Joint replaced the thing is the replacemnet U-joint isn't the correct one,( it's for a dodge 2500 )it fits but there is some "play" in the bushings.   when I called the Jeep dealer the say the u-joint is specific for that year Jeep, and nobody seems to have the part number or have the correct one except for the dealer.  So I guess my question would be with just a little play in the drive line would that make the whole car vibrate like that or could it be comming from some where else, like the transfer case for instance, but I would think that if it was comming from the transfer case I would feel that under my feet or in the shifter.  
any help would be great.
Thanks
TJ Goosley

Answer
TJ,

The u-joint must be the right one, and has to be tight.  Any play at all, will give you a vibration.  This can, also ruin bearings in the transfer case, and rear end.  I would contact some different shops, and see if you can find the right u-joint.  If not, then call another dealer.  If you can't find one that can give you the part number, then I would suggest that you contact the Jeep corporation itself.

Carl