I'm under the imperssion that the 85's have the 2.88 diff. you can crawl under the car or put it up in the air and find the ratio stamped on the drivers side of the diff case just foward of the rear cover at about the 7 oclock position standing behind the car looking at the diff. Use some brake clean to wipe the grease off the housing and you'll find it.

As far as removal and installation. First you need to be able to remove the fill plug. If you can free that up, then drain the diff oil from the lower plug. This is a precaution in case you need to keep what you got. You'll need then to refill the diff.
Once drained, remove the rear cover. The axles are held in place with circlips. Axles won't slide out from diff unless you remove them from the backing plates of the rear brakes. So you've got disassembly on each wheel to remove the axle. Once the axles are out of the back plates, Big nut in center of hub, you can manuever the axles away from the diff. Next you need to disconnect the driveshaft. Good time to renew the flex disc if needed. then there are 4 big bolts that hold the diff into the floor. Also I think there is a rear diff mount. that's about it, but do a search. I think Lawson the pilot did a real nice pictorial on the diff r & r.
Make sure the car is supported so it won't fall on you, best on a lift to remove the 4 bolts. and wear eye protection. take pix for all of us.