I'd look to the 700R4, rather than the AW4. The AW4 is an okay trans for a stock XJ, but there are some weak points to them. They make a lot of heat, the 1st isn't all that low and the OD isn't that high, the shift points are computer controlled and a little strange and the lockup torque converter does it's thing at strange times, especially if you're running anything other than the stock 215/75R15s and 3.55 gears that the XJ came with.
Here is what Novak has to say about the AW4: http://www.novak-adapt.com/knowledge/aw4.htm
They have much more favorable things to say about the 700R4, especially the later model, 4wd truck transmissions: http://www.novak-adapt.com/knowledge/th700r4.htm