I do not believe that you have to remove the engine to change the RMS on a 4.0, regardless of engine.
Where are you finding that the rebuild for the AX-15 will be $700? Or is that figuring the initial purchase of one that needs a rebuild? I'm seeing rebuild kits for $350. That is assuming that you have the AX-15 crossmember already and that you're clutch and flywheel are in good shape. I would also only do an external slave AX-15 swap, as the internal slave cylinders don't seem to be made well anymore.
How do you use the truck? If you do a lot of towing and hauling or like to drive the truck like a sports car, it would be worthwhile to swap to the AX-15. If you just casually cruise around in the truck and don't really use it for truck stuff often, the BA10/5 will do just fine. In fact, my BA10/5 was a much smoother shifting transmission than my rebuilt AX-15 is.
How much oil is the truck really leaking? Unless the oil leak is very frustrating to you (I am one of those people that hates leaks) or the clutch is getting oil on it or its causing other problems, you can buy a lot of oil for $700. If the engine also has 220k miles on it, there is a chance that the crank is worn and a new RMS won't completely fix the leak anyways.