What makes you think that you need a new ring and pinion in the differential? Until you open it up, you have no idea as to exactly what is wrong. Sure, you will have some labor to get it out for inspection, but it might only need new bearings. If you pull the axles, and driveshaft, the differential can be removed from the housing, and inspected without disassembling the gears themselves. Not being familiar with that particular year, it might even have a removable rear cover, so you can inspect it without removal. Doing a parts check on available parts, I see that the bearings and axle are referred to as GMC. Without having an old Hollanders interchange book, I can say for certain if it is just the bearings or the complete center "pumpkin". Even if you found a used one, you have no idea if it is going to be any better than what you presently have.