A heavier diesel burner would be even better.
A Detroit or Cat is out of the question for a TJ, but there are diesels that would work. A OM617 Benz motor would be ideal from an availability standpoint, but the sump is in front, so another oil pan would have to be made. Or dry sump it, but that would probably be high dollar.
Nissan, Mits, and Isuzu all have possibly suitable plants.
The four cylinder B Cummins is excellent, except it is redlined around 2200 and has so much torque the stock driveline probably won't handle it.
The Liberty comes out with a VM electronic diesel (for the US market) in 05 and maybe they will pull their heads from their ass and offer a retrofit kit. A swap using one is possible but you need all the boxes, harness, et al. The upside is power and torque and smooth idle: the downside is it's an electronic engine. There is also a mechanical (I think) inline six VM Cento engine that was used in FedEx vans and in several other apps-a German tuner was putting them in Jaguars at one time. Don't know if weight, size are ok for a TJ.