[Chrislong]

What Steve says above is important.
Also, if you lift the car so shocks are at full travel, just as the wheels leave your bench the car should not rock corner to corner.

If it does, first measure the shocks off the car - they need to match, wind in the shock bottoms to achieve this. If they match and the car still rocks then check - bent shock mount screws, twisted chassis, bent hinge pins.

Next, also important (all of this is important), are the shock springs and preload is all perfectly equal, also the rebound too. If your springs are old - change them, they do degrade. If unsure how to check this - buy the Losi shock tool, its worth having defo.

if all the above is equal, and it still does it. Tighten your diff a tad and try it, and beyond that your looking at checking the corner weights and weight balance.... and that is getting deep into fine detail (this is what wins races afterall!)