From what you describe it sounds like you have a lot of wear on the splines, both on the output shaft and the driveshaft, probably down to lack of lubrication at some point.
Perhaps Hann or one of the other Euro boys will be able to tell you about interchangability, but I don't see why not as long as you get the correct stud pattern diff it will fit.
Taking a diff apart isn't easy, and yes you do need some special tools to lock it all up etc.
Ride Safe. Skippy. QRT
GL650 Quike, CX500 z/A/B, FJS600 Silverwing, Sloath Bomber Project............