First off have you ever set up the brake controller? After adjusting all the wheels follow the road test. This generally means traveling at 25 mph on a level surface and slamming on the manual controller. If the brakes lock do it again after you have backed off slightly on the controller gain. Keep this up until the brakes do not lock on a hard manual stop. That is the set point for the controller.
If after this set up you have one wheel that you can not keep from skidding that wheel has a mechanical problem.
Keep in mind that if the trailer has auto adjusters you have to hold the adjuster away from the star wheel with a small screwdriver while adjusting the brakes. If not the adjuster will keep the star wheel from moving in one direction, the relaxed direction.
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