Wasn't Jeep going to make a conversion kit for TJs with the 2.8 CRD engine? That would be the ideal place to start.
Like I said, if the motor is Federally approved you can probably import a used one of the same model from Europe. Almost all of the KJ Liberties(Cherokees) over here are diesel. You will need one from the same model year as the diesel KJ sold in the USA as far as I know.
All of the VM engines in the Federal KJ diesels were made in the same factory as the European KJ diesels in Italy. So Jeep had to import the engines from the VM factory in Italy to produce the Federal KJ diesel.
I found the following info here
http://www.epa.gov/OMS/imports/quiktext.htm#nonusversionB. (2) Engine not in Vehicle or Chassis
EPA regulates the entire vehicle, not individual parts, for cars, light trucks, and motorcycles. If an engine is not installed, and is to be used in a car, motorcycle, or light truck, it may be imported as an automotive part. No approval or Customs bond is required by EPA. Importers should be aware, however, that replacement of an engine in a U.S. version vehicle with an engine of a different type or model year which is not currently covered by an EPA certificate or will be covered by an EPA certificate prior to introduction into commerce may violateEPA regulations.
To import an engine for use in a car, light truck or motorcycle, the importer must file with U.S. Customs, upon entry, an EPA Form 3520-1 and declare code "W" on that form.
It might take a little bit of investigation with US Customs and your State DMV, but it would be worth it if you could get one for $5K-£6K ready to install. A rebuilt British 2.5TDi engine costs about $3000(1500 GBP), I would not expect a used 2006 2.8CRD to be much more than that.
If anyone is seriously interested in going this route and is prepared to spend some time making the necessary enquiries, I can find as many complete 2.8CRD engines & transmissions as needed and ship them over. I'm used to sending Jeeps back to the USA by ship, might as well start sending parts.