This is only a wild guess, but have you considered that the other side may not be applying as it should, something out of place in the assembly, and the locking side is trying to do all the work, and the front is working normally?

Just a shot in the dark ... years ago had something similar, but on the front, that turned out to be a collapsed rubber hose, internally, stopping the flow to one wheel, and making the other do all the work.