2wd to 4wd, I think we all know someone whom MIGHT be in the process of thought in reguards to doing it. Might already have someone aquiring a trashed 2002 4x4xlt for the Tcase and front end. If you can find a good salvage vehical for 500-1000$, and have an older truck ( say 2003 or older ) , for the trade-in value and the new trucks payments, if you have the time.. Might be worth looking into. Consider having a 2002 or 2003 that has gone over the 36,000mile warrenty... makes things even more intresting.
As for 4wd to 2wd, as Bob said, the drivetrain does have resistence. The main issue I would see is changing the transmissions after pulling the transfer case, possible ECU issues/compatabilities, having to re-balance the drivetrain.. I think Trevs offer or just trading the vehical in might be the best option.