The Cummins 4BT would be ideal for a switch. A lot of bread vans and other delivery vans have this engine. The whole van can be bought, and parts not used can be resold. Here is a discussion o nthe conversion:
http://www.dieselpowermag.com/tech/0905dp_cummins_diesel_engine_swap/index.html
Here is something from the Jeep forum that can be of help when converting any other vehicle:
http://www.jeepforum.com/forum/f8/official-cummins-4bt-swap-thread-cj-7s-537709/