I think we've got all the "big" stuff covered already, but I wanted to mention that I've always wondered if a rear air deflector might help to alleviate some of the large low pressure zone generated by our brick-shaped butts at speed. Something to experiment with if you'd have the chance.
Aerodynamically, if you don't want to invest in fabbing-up a whole belly pan, there were stock front skid plates that, despite the small amount of added weight, help aerodynamics quite a bit. You'd gain a slight amount of MPG while also gaining some slight front-end protection. It's not perfect, but it's a bolt-on that you could do for a small win-win.
FastPlastic blocks-off a good portion of his/her grill. He/she has never had a cooling problem, but that still makes me nervous given the XJ's cooling system. I think I'm going to try it during the colder months though if I ever have the time to fab-up some plastic panels that I can pop in and out of each grill section as needed.
(Knowing the benefits of ZDP doesn't make you an idiot. Thinking that it is the end-all-be-all for flat-tappet engine lubrication and using ludicrous amounts of it would make you one of the idiots I speak of, though.)
Build Thread -- http://www.jeepforum.com/forum/f177/whats-chrokeese-build-thread-2525690/