Slipping can also occur if the pinion gear broke (or any of the gears for that matter) in the rear diff, so by changing the fluid he'd see metal shavings in there, then it would be pretty obvious.

Regardless of that, anytime I do my transmission I usually do my Rear Diff too... just keeps them both on the same page as far as I'm concerned.

 

You will know when a pinion gear breaks.Its diffidently not a slipping sensation..Its a catastrophic disaster..Been there done that more than once..Mine has the drain back lag just put in neutral for a sec then in drive, its there every time...

 

I've seen one have a few teeth get shaved down and it would cause intermittent slipping... wasn't pretty when we opened it up.


Wish I still had the picture of it.

 

^^^ What he said, and even if a few teeth break, you will still definitely hear it