Think Diesel Bob (
http://www.dieselbob.co.uk/index.asp) does them for the 2.5TD, it's all a bosch system so will come under that rather than Jeep.
The XJ TD has a seperate ECU for the engine and the body control system, so it's not as straightforward as most, but you should just be able to upgrade the ECU/Chip with too many concerns.
However, you need to make sure it's from a reputable supplier as if it's not quite right (fuelling etc) you could do more damage than good. Not only that, but to make it worth while you may need to change some parts to higher spec ones to get any real benefit. Diesel's use air flow as the trigger for the fuelling, so you'll need to increase the air flow, otherwise you'll just be burning more fuel with little benefit, i.e. lots of black smoke but no real oomph. then there's the injectors. If you're adding more fuel you may be reaching the limits of the injectors flow capacity.
It really isn't as simple as just shoving a new chip in...
