Vibration

hey all I have a 95 grand cherokee laredo....has 4 inch lift W/ 31 bfg's.....Ive had it for only a few months....it was built up by a very good mechanic and very well maintained......my only real problem so far is in the last week or so I ve developed a vibration....not wheel bearings...not u joints....seems to start vibrating around 50km/hr(25 to 30mph)if I back outta my gas the vibration stops....go back into the gas it starts again....if Im coasting at speed with no foot into the gas no vibration either.....only when Im into the gas above 45-50 km/h....if Im doin that speed and drop my trans into 2nd(automatic) the vibration does not speed up(as that would increase driveline speed and not wheel speed)....Im baffeled .....I own a 75 cj 5 aswell so am very familiar with trouble shooting the wheel bearings and such......I need help!!!! It's driving me nuts!!!!!!Please drop me a line if you have any ideas or suggestions.....thanks  
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Sounds like drive shaft vibration.

If the drive shaft has been hit and dented, the vibration will not change with loading and coasting.

But the load on the u-joint and the u-joint angle will change with loading and coasting. So, I'd say it's your u-joint.

Your drive shaft will not vary in speed when you shift the transmission. The rpm's will go up or down on the engine, but the drive shaft speed is in a constant relationship to road speed regardless of transmission gear position.  

I had the same problem with my Wrangler.  To check the for a bent driveline I put the back axle on jackstands and had somebody turn the tires while I was under the vehicle watching the driveline.  I could see the driveline was out of true.  Good luck  

That and tie rod ends can tend to do that too, but its more of a shaking at that point.  

ok guess I shoulda put up that I got this prob solved...was front u joint on rear shaft ....was no freeplay when installed but when off and in vice I could feel the freeplay......$40 fix.....  
http://rides.jpmagazine.com/ride/1133684/13flames/1975/jeep/cj5/index.html