I have this clunking noise coming from the rear of my 97 Explorer. It started off every once in a while. Now it happens all the time. When I turn left it acts as though the rear tires are locking. I took it to the ford dealer, and they told me it was the internal gears on the transfer casing and that the part alone to replace is 2600.00 Does this sound right

 

No. Something sounds wrong. If it's the transfer case it would lock up in either direction no matter what. It sounds more like a problem in the rear end gear cluster. The price on the complete transfer case would run about that price. The only thing the transfer case even does is add a chain (huge) to the normal drive shaft tospin the front drive shaft to the front axle. If you have a problem with it the rear wheels will usually either spin or not. If it ONLY locks in a left turn it's likely the gear cluster in the rear and you'll end up with two bills.