1997 Lincoln Town Car Differential grinding vibrating and noise

Having trouble with the differential. It's leaking and uphill or at low rpms it grinds and vibrates making a loud noise like it's not catching or its struggling. I had the transmission checked and that's not the problem they said it's the pumpkin. Was wondering what could cause this and how to fix it and also was wondering if it were possible to swap it out for the diff of a 03 mercury marauder without serious complications