Transmission slips when shifting from 2 to 3 going down hill.

[f5fstop]

Are you in 1st or 2nd when doing this shift into 3rd? I'm going to guess that it might be normal, but haven't tried it on this vehicle.
If in 2nd you are jumping from 1.625:1 to a 1.000:1 trans gear ratios, and you have a 4:1 gearing in the t/case. So, the jump could be due to the higher gear you manually selected.
I'll give it a try in a few weeks out in Moab, maybe I can wake my kid up.

(Let me make note, I'm not a trans expert; not enough electrons running through the case for me.)