I have to agree with Mr. Shiftright.
If the rear differential pinion seal is what we're talking about, then the parts involved are;
pinion seal - around $20
Pinion crush sleeve - approx $20
Differential oil SAE 75W-140 High Performance Rear Axle Lubricant XY-75W140-QL or equivalent meeting Ford specification WSL-M2C192A. Approx $30
Additive Friction Modifier XL-3 or equivalent meeting Ford specification EST-M2C118A. Approx $15
Approximately 1.7 - 2.3 hours labor
As you can see, that isn't even close to what they are quoting you for. I'd have to see a detailed quote to be more specific.