How do I stop 4Runner tire wear on inside of drivers side tire?

Really sounds like you need the front end rebuilt.

What is the fix if it is the rear drivers side with the problem (tire wears only on the inside)?

Bent differential.

Before I spend a few thousand replacing the rear differential, is there a way the repair shop can check it to make sure it is indeed the problem?

Any good alignment shop can do the measurements and tell you. However you may not need a replacement differential, a good machine shop maybe able to heat, realign the tube then temper the tube. Hope this helps on Christmas Eve.

have someone follow you as you drive it to see if it is dog tracking ( meaning back tires are not following the front tires. ) I had this problem and found it was a bent trailing arm the rear end connects too. The alignment effort should reveal this as well .