Here is something to check. Some manufacturers mount their jacks so they run parallel with the frame. You either stand directly behind the trailer to jack them down or you stand directly to the side to crank then down. This puts the jacks all facing the same direction and they tend wobble. The jacks should be mounted diagonally in each corner. This reduces wobble since they're all opposing each other.
If yours are already mounted this way, I would just replace the rear with a one piece jack, like the
BAL jack. This may be a more reasonably priced fix.