Just had my 07 Classic Siverado into a dealer for warranty replacement of a leaking transmission cooling line crimp fitting. I have only 30,000 mi. on the truck. They suggested changing the rear differential fluid and transfer case fluid at a cost of $340. I asked what they based this on when the book has no interval. The only justification was that they thought it was a good idea. Am I getting taken for a ride? I did pull a 10,000 lb. horse trailer mostly on the hiway for approx. 2,500 mi. of the total mileage. I have been conservative with the Allison transmission and flushed and changed at 10,000 and 25,000. Thank you for any advice.