Icarus-I've heard about a drop at the most (if not running, do NOT have it running), just place a rag underneath to catch if necessary. The whole swap took me about 10 minutes overall. The key is to take your time and not wrench on the harness plug when removing.
Get yourself a nice 11 1/16th deep-well socket (at any chain store) and a 1/2" ratchet. The guy at the store I went to suggested I could get it out with a crescent wrench, but I highly suggest the ratchet as you can get a nice, secure grip on it for removal and re-install.
I do not have an A/C unit on my TJ and was able to take care of business from up top, I've heard and read to attempt from underneath otherwise (food for thought).
Again, when in doubt take your time and make sure you do not cross thread the new sensor. Doing this (as with everything else you screw onto your engine block)=bad times