rear end question

I have a 95 Z71 and at only 60mph or higher when letting off the throttle i t vibrates like i'm running over those strips they have on the road before a stop sign. Does not do it in town, or driving at a constant speed on the highway ONLY when i take my foot off throttle at highway speed but it seems to get better the further i drive,(75mi on highway it seemed better) any suggestions? Also when I am going at a constant speed in town or on the highway i here like a hollow or "dry bearing" like sound coming from rear end. I put new 80/90 w/ a qt of lucas and it still does it. I think they r related. any ideas.

I'd be looking at u joints here also.

Do what maxwedge said 'cause thats the easiest, but if it persists, check your axle bearings.