The Bilstein's might be cheaper but the Koni's ride is superior...by a long shot. If you get a chance (and the $$) to change all four out, I highly recommend it. They can be had for around $125 ea., sometimes less depending on the deal and circumstances.
Re: the air springs (air bags), leveling valves etc., even if all are well, there is a correct ride height for your chassis that will make a marked improvement in your ride quality and handling. If the ride height has been improperly set or needs adjustment, it would be a good idea to pull the driveshaft and check the condition of the u-joints (internally) for lubrication, scoring or disintegration, as well as the slip spline on the shaft. A DP has a very short driveshaft between the trans. (Allison usually) and the engine and a little change in ride height can translate into a very big difference in ride and handling...plus you wouldn't want to damage that $12K - $15K tranny.
While the service techs are "under there", have them check out your sway bar bushings, brake slack adjusters, rear differential fluid quality/level, wheel bearing oil level, steering linkage and bell crank adjustment and lubrication etc. It might be $$$ but a complete chassis inspection and service will give you a lot of piece of mind going down the road.
Too bad about the Sun City thing as I do all my own maint. and it saves a bundle of the hard to come by $$$.