Air Bags for 2004 GMC Yukon Denali?

100,000 miles so the rear shocks may very well be toast. I just had my mechanic check the fronts and he said they were fine, sounds like it may be time to have the rear shocks checked out. I suspected this was the way I'd have to go, but wanted some other opinions since it's the more expensive route.
Yes, Gary, the compressor runs when the trailer is attached. It runs quite a bit actually, which makes me fear there's a leak somewhere.
Thanks for the replies guys, I appreciate it.