Heres a chuga chuga, thunk thunk question which is driving me nuts because I can only hear in while accelerating.

I get a high pitch ringing/whine from the back end under load. Cruising ok, but when I floor it just a bit the ringing begins.

Could this be an angle problem of my rear diff caused by my 4'' lift, rear seal binding, out of balance drive shaft, or something else you have come across which I have not?

Thanks for any replies, ideas in advance, as I don't think anyone would volunter to lean over the back gate for me while i motor up a hill..urhm, any takers?