[Kevin Sevcik]

The motor he's talking about is the (dearly departed) van door motor, I'm assuming from the fact that it has a screw thread on the end. It's no longer legal anyways, so you'll have to change it for a different motor, but for informational purposes it's a worm-gear transmission and it is, in fact, backdriveable. I could go into all the technical details, but suffice to say, worm-gear transmissions can still be designed to be backdriveable. It simply depends on the lead angle of the worm-gear. If the lead angle is less than the friction angle, the worm-gear won't backdrive.... usually. This usually works out to a lead angle of around 8 degrees.

At any rate, to answer your actual question, I don't think there's a non-backdriveable motor in the kit that will help you. The only motors of this type in the kit are the window motors. They have a stall torque of around 9 lb-ft. That means the center of mass of your 6 lb arm will have to be less than 1.5 feet for your unmodified arm to be able to lift itself from a horizontal position. However, as someone here noted, counter balancing your arm may work, as would gearing down the window motor.

In summary, your current motor sounds illegal. Your options are to switch to a window lift motor and either gear it down enough to move your arm, or counterbalance your arm enough that you can directly drive it.