The rear differentials are broken 1993 jeep grand wagoneer.

Your differential has several parts.....you have a ring gear and pinion gear (matched set) and a set of spider gears or a positraction unit. Those are known as the center section. you also have a pair of axles. Of course those parts all have bearings too.(except for posi and spiders). The housing is also considered as part of the unit.You need to know what has broken. The ring and pinion gears MUST be set up by an experienced technician as they have precise settings and gear contact patterns that if not correct will cause immediate noise and eventual premature failure. Any time an inner pinion bearing is changed those settings need to be made again as well. Spider gears are easily replaced with no other work but you need to clear all debris from the insides to prevent bearing damage. Axles are also replaceable. Gear sets are expensive. Sometimes it is far more cost effective to replace the entire unit with one from a scrapyard. In that case all you need to know is the exact gear ratio of the one you have. (sometimes stamped on the edge of the ring gear or on a tag on the diff cover, or found by dividing the number of ring gear teeth by the number of pinion gear teeth. You will get a number like 3.23, 4.11 etc. (examples only).
I hope this helps you understand what you are dealing with.

Look for a used replacement on ebay, craigslist, or junkyards. When you find the parts, then find a shop to install them for you.