You can buy a lot of gas for $20,000. That said, you could find a used (Ford) Cummins engine with the matched Ford transmission out of a F650 or similar to swap at a junkyard for much less than $20k.
The Cummins is only paired with the Allison 5 or 5 speed, or with one of the Eaton or Spicer transmissions (inc. Eaton auto-shift). It is not currently available with a Ford transmissions. I'm not sure if it was in the past. I don't think any of the F650 transmissions would fit well under a van. They are all huge compared to the van/pickup transmissions.
Thanks carringb, if I were to do the swap I would probably try to go with the allison trans. anyways-but I'm not planning to do any swap anytime soon, just thinking about it. But I have a question for you that's off topic, I noticed that your van is a dually- I would really like to put a dually in the back of my van but then my sliding door would hit- do you have the double doors ? Any idea's on how I could put a dually under the rear of my van and still use the side door ? See my van has a snow plow that was installed by Pathfinder and a dually rear would greatly enhance my traction as well as make it much more stable at highway speeds. Would it be possible to switch from a sliding door to the double doors instead ? Any idea's would be greatly appreciated, Thanks !