Front axle seals

Had my 03 Yukon XL in for service and they said the front axle seals are leaking. I've never seen any oil on the ground where I park it.

Other than seeing the oil on the ground, how would one know if they are leaking? Would it affect driving?

You would look at them. If they are wet they are leaking. When the axle spins is flings the lube in all directions. some will stay on the vehicle and some will end up on the road you drive on. This is a known issue with these axles and a fairly common service/repair.

Other than looking, how would one know that there is a problem. In other words, if the mechanic didn't spot it, I'd be driving around not realizing there is a problem. Would there be any other signs before the differental get's ruined?

Not really.

When researching this, I've come across different terms.

Are an axle seal and an output shaft seal the same thing?

The axle seals would be the seal that the drive axles seals into on each end of the differential. The pinion seal is the seal the drive shaft seals into on the differential and the output seals would be in the transfer case where the drive shafts go in.

To close the loop on this post - I took the truck to an independent auto repair shop to get a second opinion. Both front seals where indeed leaking.

He replaced both for $380. The dealer had quoted $600.