2003 Yukon Rear Axle

OK, I went to the dealer today for a front end alignment and transmission service (flush) on my 03 Yukon with 75k miles. He called later to say I needed $650 worth or rear end work b/c the "seals were leaking". I told him I would look at it later as my wife was picking it up.
Upon inspection at home, there is some evidence of leakage around the center axle hub to both the front and the rear. This vehicle has always been garage kept and never--I repeat, never, has there been any rear end fluid leakage seen on the garage floor in the 4.5 years we have owned it. Again however, I can see where there has been some weeping to both the front and rear of the center of the rear axle hub.
Is this normal or does it need addressing?
On the phone, I asked the dealer if he had checked the level of fluid in the axle and he said that he couldn't unless he dropped the pan. When I looked at it tonight, it appears there is a check screw on the top of the axle. Isn't that what it is? Can anyone tell me the correct procedure for checking the level?
Can I simply replace the axle gasket by removing the screws, plate, and inserting new gasket with silicone, then refill with fresh fluid? What is the correct fluid?
Thanks in advance for any replies!
Sincerely,
Ken

Yes the fill plug on the side is where you check the level of the fluid. Changing the fluid requires removing the cover, installing a new gasket and refilling with 75W-90 Synthetic fluid. Check owners manual for amount needed. I think is 2 qts plus another 4 or 6 oz.

Some moisture on the pinion seal is normal but if it leaks to the ground, the seal should be replaced.

//2000CAYukon