Madmax - are you lifting cars with the rubber pads meant to lift with or lifting them on the arms of the 12k, because whether you're lifting cars with a 12k Rotary, Challenger or Bendpak, Atlas.... you're not going to be lifting very productively.
as far as curb weights (not GVWR):
http://www.gmc.com/sierra/2500HD/specsCapabilities.jsp
you can lift some smaller cars on a 12k but not very productive, unless you have time to shuffle vehicles forward and backward to spot (unlike a Challenger CL10 with the offset arm design)
rgullett83 - if the heaviest truck (yours weighs 7100), you're fine with a 10k lift (make sure your load is spread correctly across the arms 2500 ea arm) and you leave yourself the option of being able to lift your truck and smaller cars the right way.
http://www.challengerlifts.com/versymmetrictech.shtml
http://www.challengerlifts.com/CL10series.shtml
I have 2 12K BendPacks and they lift cars with no problem. I have lifted Hondas , Mercedes, Jaguars, Caddy's and you name it. Don't see how you say they are not productive. I use the rubber pads for cars and frame captive adapters for the trucks.