We were told the rear differential(drive shaft, the part on the inside, not easily accessible) in our jeep has a leak. Is this a common occurance for a jeep with 85,000 miles and is there a reason it took a repairman at Sears doing a oil change to point it out after it had been in the shop at the jeep dealership for a thermostat problem and never noticed. What should we expect to pay to have this fixed. Thank you for your time!

Laura