so you diff. had water in it from not being changed after water crossings, understand able. But the .75-1 inch difference in length from an 99-02 & a 03/04 drive shaft was the cause of your vibration?

Apparently from the diagnosis, the diff was part of the vibration problem and the drive shaft was the rest of the vibration problem. He rebuilt the diff first and it lessened the vibration but there was still some vibration. He then put another 02 disco's driveshaft in mine, as a test, and the vibration was completely gone. We ordered a new driveshaft, correct year, and installed it and all is good.

It is possible that the issue with the driveshaft had nothing to do with the difference in length but that this particular one was not balanced properly or otherwise defective.