He did just change his filters.
I think there's air trapped. I'd lift it off the ground and run it a long time. I've seen other members have to do that after draining the oil. The new oil will foam, and you have to work the air out a little at a time.
So, you lift it and run it for 5 min in forward. Then shut it off and let it sit for a. Hour or oven night. Then run it reverse for 5 min the next morning, then wait till evening and try it again...
The process was something like that anyway. Wait for others to chime in.
Jeep - have you thought about just pulling the drives apart and replacing the seals and backing rings?