How much do you want to spend?
If you want one that's really bullitproof behind a 4.0, try an Allison 1000 transmission. You'll never break the transmision. If you really want to go hardcore, they have modified ones for more durability as well the diesel guys send 600+ horsepower and 1,000+ ft lbs through.
I wonder if it'd work with the computers and whatnot... if I bothered with the Allison 1000 then I might as well swap in a dmax as well. Gosh that'd be a wierd XJ.