I've ran into the same problem here and there with my Nitro rustler over the years and I chalked it up to the diff being the problem. The grease I was using wasnt viscous enough and would cause the diff to favor the one wheel. I've noticed my 3.3 Rusty likes to "walk" when in wheelie mode where it hops from one tire to the other.. I was going to cure that problem with longer suspension arms from a Jato. A buddy did this on his N.Rusty 3.3 - he used Jato driveshafts and Jato rear arms and he said it was a totally new beast. This could be something worth looking into.