whenever a friend gets a new xj (usually craigslist specials) we take it to the car wash in the ghetto that has engine cleaning bays, bag the usual suspects, and hit her with the engine cleaner they have, works like a charm. if your starter or sensors are on their way out, it will excellerate the process, convieniently letting you know what you need to replace!! not for the faint of heart.
it took installing 2 lift kits on 2 beaters before we started cleaning the suspension/underside with the same process, it makes the lift process soooooo much nicer! if you take a set of ramps w/ you, even better!
ive killed a starter though, so be forewarned to take your AAA card, and dont have to be anywhere that afternoon.
take wd-40 w/ you, incase you get waterlogged. after you get home from the cleaning, get under there and look for any fresh leaks.