Along with Bernie's suggestion, take a look at the jacks.
Preferably on level ground, measure the distance from the ground to the bracket welded to the frame, they should be nearly the same with all 4 jacks.
Look to see if all 4 jacks are bolted on in the same relative position.
If all looks good, height and jack placement, my suggestion would be to lower all 4 jacks one bolt hole. Lowering only the front or rear can introduce unequal loading on the frame if one set bottoms out before the other.
In other words, on flat level ground (parking lot for instance), lower the jacks till they just touch ground. The exposed ram length should be nearly equal.