Just want to thank everyone here. Like X5Tanya, I have the 01 4.4 with only the rear air springs. Reading all this helped me diagnose that they were leaking. Found them on eBay for $199 for the pair, lifetime warranty. Replaced them myself in about 2 hours. Easier than changing brake pads. Just need to be sure to jack up both rear wheels at once so that suspension drops enough to have room to get them out. Work only under a vehicle that is safely supported! (Or pay a shop to do it.)