I have the original 3.9 out of my 94 D90 available for sale. It was running a little hot before I removed it and could have a bad head gasket or something. I took it to the dealer twice and they said that it was fine and that they could not get it to overheat. Since I'm no mechanic, I bought a used 3.9 and had them swapped.
The motor has 60K on it and is complete, with the alternator being the only exception. It runs very smoothly and never had any problems until the overheating thing started. If you are interested or have any questions, contact me at: firstname.lastname@example.org
I 'm asking $750 OBO.