[40cpe]

We called it dog-trot in our neck of the woods. You say your frame is straight, a bent frame could be one cause. Another possibility is the rear axle isn't sitting square with the frame. I would first measure from the axle to a point on the frame that can be identified as the same on both sides, such as a rivet. Then check that the tie bolts in the springs are not broken and are situated in the hole in the spring mount. There could be weird things, such as someone installed unequal length springs. The measurement should tell you if you have a mis-located axle.