Maybe it is just semantics, but I don't think that your gearbox (the sort of flat, plastic case attached to the motor) is bad since you say that the motor side leg works. Rather, from what you describe I think the problem is in the bevel gears and rollpins inside the non-driven leg. See the drawing on page 3 of this manual from Venture found on the Heartland Tools tab at the top of this page. They list a repair kit which includes all of the parts in Red. I think you want the 999-1 kit that includes the follow drive shaft, which should be the one on the non-motor side.
http://manuals.heartlandowners.org/Suspension/Jacks/Venture/Langing_Gear_Manual.pdf
I saw just the bevel gears on Amazon for $26, but they were listed for Atwood, not Venture, but I suspect they are all the same. An RV parts store might have the answer. The Lippert website has the complete kit for $56, but it is listed as the Lead drive shaft kit. You might also contact Dormar industries. I have spoken to them, and although their website doesn't list the repair kit, I bet they have it, and they are much cheaper on the complete leg kits. They say that the make leg kits for Venture. Here is their link:
http://www.domarproducts.com/index.html
Good Luck!