I say, clutch, I'm now thinking Diff. Drive has a slight incline, reversing straight at low revs and not much clutch, silence. Hard lock - woomph, woomph, woomph!

I guess at full lock the rotational difference between the wheels is at it's most. I've read quite a bit online about changing the oil in the diff can get rid of noises. I'm guessing it's garage time, whatever.