Check your bearings, one could be bad and causing it to lock up. I'm with Rooky on the trans issue, tear it apart and see what's up with it. I'd recommend upgrading your idler gear while it's apart, I went through a few of them before Traxxas released the Nitro Slash with a steel idler gear...
I actually got rid of that box, it was a spare from my LST but I didn't like it. The roll bar is sweet though, but it's from New Era Models and they went out of business....so it'll probably be hard to find.
I love my Rusty man, it's a fun little basher and is super easy to work on. I have gotten it to wear it can really take a beating, now I have to worry about cracking my engine block (I've done it a few times). I prefer the T-Bone Racing bumper to the RPM, I broke one of my RPMs when I clipped a...