to mimic a differential preload you could add/subtract a certain amount of torque to the differential output in UFGearAxle::getLockedDiffTorque.
I have no plans to do major simulation changes in the fgear 1.x branch, I just don’t have enough time right now. when I get the time my plan is to work on a possible 2.0 rewrite but that is unlikely to happen in the short term.