http://www.drivingline.com/articles/39l-cummins-engine-4bt-diesel-pros-cons/4BT / BTA are HUGE. Like nearly additional 400 pounds HUGE.
and with half the horsepower
and 70% of the torque AFTER cranking them up to 'bomb' AND adding intercooler and exhaust.
They are tough though. And easy to get parts for.
The Cummins repower 2.8, however, is aimed directly at Jeeps. Both diesels and otherwise.
Still pretty gutless stock, but you don't have to fight with FCDJA to get a good tune, and you'd have great domestic customer support.
One would have to be pretty married to the Liberty to put another 8k-10k into it, but once running, it's probably the last money you'd have to spend on engine stuff.
The rest is still a Jeep. Tits, warts, and all.
EDIT: Then again, if GDE starts doing combined tunes to make them work in place of factory CRDs..transmissions... That could really sweeten the pot.