Driveline noise/drone/vibration

[Big_John]

How old is gear lube, what weight is it. Fsm has quite a few notations that any gear noise should be followed by changing gear oil to rid of contamination and fill with recommended weight and have customer retest.. My differential builder recommends 85w/140 I know the factory typically recommended 75w/90.. worth a shot.

Valvolene 80W-90 with Ford LSD additive. Fresh this spring.

I had Triax 80w-90 in last year after I put the sure grip unit in. I had some awful chatter that was eliminated with the change this spring.

It's funny... Just today I remembered a friend from years ago talking about selling used cars at the dealer auction. Sawdust and bananas was the rear end fix.