Sounds like the wrong tire, and the left rear wheel is binding.
whenever I use the stock gear differential, I remove the rear left wheel with the small black adapter to key on the axle. I sand it lightly where the adapter would touch the bearing, then check the fit. I stop when I can see just a little play back and forth of the axle when the wheel is fully pushed on the axle.
this allows you to tighten the left rear wheel completely and remove slop from having a loose nut holding it in place. Improves consistency and speed by reducing the possibility of drag on the axle.
I also apply some grease inside of the differential as suggested above. Light grease can make a huge difference in the smoothness of the car and the differential unloading abruptly over any bumps. With this done, the stock differential can perform very well and even rival a ball differential in stock motor classes.