You will need to get at least a new tail shaft for the tranny if not a new rear ouput shaft. It might be easier to go get a rebuilt or junkyard tranny (170K is a lot) for a 4x4 cherokee.

Your right the front end will just bolt right in.

If you have the time, pull all the plugs and look for oil, and run a compression test.

BJ I've seen oil in the coolant. If the break in the gasket or crack in the head is right next to an oil pressure passage, the oil will be under more presure than the coolant and make its way in.