If you want to do it go for it. I myself am not a big fan of the 4.0L. It is just an OK motor in my book. Check out this link and talk to people who have actually done the swap themselves. Lots of second hand info out there that is not always accurate.
The engine compartments are different, but that can be overcome by swapping in the front grille/rad support from a 4.0 equipped vehicle. The firewall may or may not need a hammer mark at the back of the exhaust manifold.
I'm currenly working on swapping my 4.0. HO into an XJ that was equipped with a 2.8. The firewall seems to have enough clearance but at the front grill support I need to drill all the spot welds on both and weld on the one that came from the 4.0 truck. The center hood latch wont work with a 4.0 since the radiator is so far forward so you'll have to swap the hardware from a 4.0 equipped body as well.
Otherwise its straight forward, the 4.0 mounts bolt to the framerails and the crossmember lines up perfectly.
thanks for the info guys, it gives me something to think about. sounds like a lot of work and since this is our only vehicle i'll probably have to put it off for a while. dont want to sell either to much done to this one to start over at this time.