A front diff is SUPER expensive to buy or rebuild so you're gonna just have to back out of the RCD deal.
Honestly though it would probably be cheaper to buy a used diff assuming yours really is blown up. You can find used ones on Ebay, here, other ranger boards, and the Explorer boards. Usually run a couple hundred bucks.
Take a good look at your diff first though because the rangers have a reputation for a leaky pinion seal on the front. I've seen a few instances of the actual RTV sealant on the diff cover let go. I would look into those two things before you decide to have it replaced or rebuilt. The pinion seal and the diff cover seal are both cheap and easy fixes.