It could be a numbers of things. I would first look into the rubber driveshaft couplers. Perform a drop drain of the trans fluid. And these trucks are notorious for valve body issues.

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E53 X5 4.8is-Dinan S2-Street Brawler
E70 X5M-The stoplight shocker
AMG E55K-539 RWHP, 629RWTQ-my toy
GX470-the unbreakable truck.
F30 328i-JB4 STG 2. Daily MPG Work Beater
02-WS6, 6sp my other toy
...other stuff parked around here somewhere