Sure grip swap

I may be trying to read too much into this. If what you want to do is to put a SG diff into an existing 489 case with 3.55 R&P, no problem. It is exactly as Inertia says.

If the existing case is not a 489, then the R&P from the existing case will not swap into the 489. The pinion sizes are smaller and the center case length is different which may call for a drive shaft swap, too.