According to copy of owner's manual I have, the front differential(transfer case), along with the rear differential fluid should be changed every 45,000 miles:
As far as fixing the vehicle or trading it in, first, I would recommend shopping around on price for the clutch repair. $1900 is on the high side. You should be looking at $1000-1200 for this repair. Most independent shops have lower hourly labor rates and their techs are as qualified as the dealership, so you should be able to save quite a bit by shopping around.
On the transfer case, you may want to try having the fluid changed first and see if that quietens it down. Even with the whining, you may be able to get another 50-60,000 miles out of it before absolutely having to have it repaired.