just recently started,a snapping sound in rear of 2000 silverado when backing out of driveway,have also heard it in drive under load.I replaced u joints a year ago.any ideas.
thanx

 

Don't we have locking rear differentilas? That's what you might hear.
I'd start with changing diff fluid asap.
On my CR-V, brake pads used to reset themselves with audible clunck, when backing up, due to change in direction of rotation. But not under load.

 

If you have a GM locking ( gov lock ) diff it might be failing . mine did this before it failed ( destroying the r/p gears in the process) ( have to have factory manual for check proceedure ) if its makes snapping noise while turning then it is the diff unit what happens in the locking govenor breaks loose and falls into the gears but it locks up the unit solid first.
also if you have rear drums it can be them adjusting up but thats more of a clicking sound unless they are really loose them its time to adjust them manually but they would also grab hard going forward too especially when cold .