OK-first-throw a small rubber band or the little ring from a balloon around the ball end and the carb throat so it stays closed when you let go of it. (no servo attached right now)

Now make sure that when the truck is off and the linkage is attached to the carb that EVERYTHING moves freely. Wiggle/shake stuff around a bit just to see if anything could possibly catch.

Do you have another engine as a whole you could drop in there (running) so you could see if it's the carbeurator?

Ooohhh yeah I had something like this happen, just remembered- check that your servo lead is not touching the spur gear or it may have gotten chewed up and now when it sits(not running) there is nothing making the servo short out, but when it rolls it completes the connection and goes wot.