I would start by taking a voltage reading at the input of each magnet and both sides of the trailer plug and at the controller output. If it is the drivers side front wheel that is locking up it sounds like you have a voltage drop problem.
If you see a drop of more than 2 volts through the system I would start by upgrading the wiring where the drop is most noticeable.
Another point to test for a drop would be from the magnet output to battery ground with a piece of #12 or larger wire leading to the battery. Any voltage reading here is a problem in the ground path.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles