Check LMC Truck
http://www.lmctruck.com/
I've been told the gears are easy to break, and that this happens a lot when the rubber seal that the window fits into gets old and shrinks, keeping the window from easily rolling down, so more force is used trying to get it down and the gears get stripped. So maybe check that out and order new rubber too.
I could use one too. In the mean time, I took the cover off the tailgate, disconnected the window from the arms that hold it up, and ran a few pieces of heavy wire around the top of the tailgate through the little brackets attached to the bottom of the window and twisted together the ends of the wire. This holds it up for now, and if you need it down, just untwist the wires and GENTLY let it down.
It's not very convenient, but it works for now.