I have been hearing a decently loud ping or a quick grind when my truck is in reverse. I am now hearing it in drive as well. I believe that it is the transmission but the transmission shifts just fine and there is no real pattern to when my 1997 yukon gt makes this noise. (Now as frequent as when I get on the gas a little and even when I coast at about 20mph...so everytime I take the truck out it makes this nasty sound) This noise happens not when I change gears but when the truck is moving and only when the truck is moving. ( weather it be forward or reverse).

I am going to put the truck on stands today and turn the wheels to see if I can locate the exact spot but this is a weird problem.

Also I have eliminated suspension and the transfercase (well at by eliminate I mean the transfercase works and the suspension is quiet)
Any help or thoughts would be appreciated.