The factory stock 4.88 diff is weird maybe, not really 'rare'. The diff takes a different ring and pinion than yours. There is not really a downside to it though, if it is in good condition, go for it. You might want to replace bearings, but that makes it as costly as redoing your own diff. You won't (most likely) be able to regear your diffs for ~$300, unless you do it yourself - which I would not recommend without some careful mentoring and special tools.

To R&R a diff, you only need FIPG. The gasket is worthless. I use Toyota driveline FIPG (orange) but there are a number of good brands available.