i have a 91 camero when u are driving it and u let off the gas it makes a grinding sound in the rear end what could it be?

 

could be the differential gears

 

Worn bearings. If your pinion gear bearings are sloppy, the pinion gear will want to climb the ring gear, giving a bad gear mesh. Or, your outer wheel bearings could be worn and putting some up or down force onto the ring gear carrier (which could also have sloppy bearings). You didn't say, but should we guess that you have a lot of miles on the car?

 

It might be the e.

 

Rear U joint? Brakes ok? Differential fluid level OK? I'd jack it up and get the wheels moving and see if you can isolate it from there.