Regrettably, the P38A doesn't have fully floating axles lke the classic so you'll not be able to do that.
Very different. The only similarity is that the diff ratio is the same at 3.54:1. Nothing else is the same - even the number of splines differ. An '83 model is a 10-spline diff.
I wonder if you have a seized VC. Hmm, maybe not if the fronts don't move at all. What happens if you jack both wheels off the ground at the same time? With one wheel on the ground it is quite difficult to rotate a the other wheel against the VC. If the diff is rotating OK, then with both wheels off the ground the wheels should rotate OK.

