Sorry I was late to the game here. Rear axle seal leaks are fairly common on Chevy rear axles, some as early as 40K miles. The shoes will need to be replaced, and the drums thoroughly inspected and cleaned.
A quick test for this is to set the parking brake, put the car in drive and give a small bit of throttle. If the diff fluid has leaked into the drums, the parking brake will not hold, even with just a bit of throttle. This is an easy repair, and any competent shop should be able to handle it.