Well In our XJ I havn't had any problems with the tailgate closing - but I do get an annoying squeak from the front drivers door seal which I al told is partly attributed to the flexing of the monocoque frame.
Also in Jan04 Jp they have a writeup on installing a stage II Chassis Stiffner to their Project XJ - I don't know if anyone is interested but check out their website:
www.tandjperformance.com (they do a lot of Jeep speed stuff too
)