GMC 2500HD Rear Axle Shaft Oil Seal
 

Took the truck for a VA inspection. They have found a leaking oil seal.

Their estimate is 60 for parts and $400 for labor (I believe that's to replace both seals), plus they wanted to do some "rear differential flush for another $116 (82 parts, 35 labor).

It is expensive, and I wonder if I should do it myself.

1) do I really need to replace both?
2) Is the "flush" necessary, or some marketing gimmick?
3) i had a horrible time replacing a tire recently on another side (it was rusted to the hub), and the truck was used in the past as a plow truck, and I know there is a ton of rust at the front breaks to the point that they couldn't take the caliper off to get to the rotor to replace it, given these are rear tires, should I have more success?

Or should I shop around for a better deal?

The instructions to do this don't look terribly complicated in general, it's the rust I am worried about. I also don't have standing jacks :)

Truck has 125K miles.

I think replacing both seals is not a bad ideal with the milage and conditions. They don't need to flush the diff. I doubt that's what they would do anyway but charge you for it. Just change the fluid and check got any metal in it. If there is metal in the fluid then you need to flush it clean. 125,000 miles is not bad.

What rear end is in it? a floater or regular 10 bolt C clip style axles?