It might be issue with synchronization. Try using one of the MMT functions to get transform of the lower arm. “MMT Get Transform -Comp” reads transform directly from the rigid body and should get proper coordinates even if “rendering” objects are not synced yet, it has option to use Socket too.
With Skeletal meshes it’s weird when they are updated, they seam to be lagging by one frame behind the “root” body, as if they are synced at a different time, then the rest of the components. I had this problem with the Hovercraft air cushion skirt. When you move them in local space, you immediately see changes as “lagging” transform of the parent doesn’t effect then anymore.
You should be able to drive it with closed differential, it would be kind of like when on 4WD jeeps it’s switched to “maximum grip” mode, when you want to go over mud or some boulders. But usually you drive very slow.
If you start skidding too easily during turns, as wheels can’t spin with proper velocity, just set higher coefficients of friction for Y axis. This won’t be realistic but at least something.