OP,
Several things are off here.
First you took the truck to what I assume is an all in one repair shop. Most places like that know very little about 4Runners.
They diagnosed it as the rear diff but for you they aren't able to do the repair. Take it to a driveline shop or somewhere in NJ that is good with Toyota's. Or even the dealer for a quote.
Also, we have a local sections area on this website. Go there and post asking for mechanic recommendation in NJ area.