Wierd Vibration after 5.5" lift install ??

[ResqExplorer]

I recently installed a Superlift 5.5" supperunner lift in my '91 with 33x12.5x15's. I noticed driving home from the shop a wierd vibration/noise, coming from what seems like the rear of the truck. It seems to happen at about 30mph and above 50mph. My first thought was that my muffler or pipe was contacting the frame somewhere... not the case. I checked my "U" joints... they feel ok. Anyone have any ideas???? I thought about shimming the rear diff. but not sure.
Please help... this shaking sucks!!
Thanks.

 

[Clint]

that would be my first guess is to put shims on the rear axle for a better pinion angle............ the vibration is probably your u-joints or the d-shaft is too short