2002 F350 ABS brake issue w/ gooseneck
 

I have a 2002 7.3 L Lariat crew cab, dually, long bed F 350 w/31K miles on it. I use it to haul horsetrailers. I use a Prodigy brake controller. It has air bags on it. I recently got a Featherlite 32' (deck) gooseneck 4H LQ that weighs > 13K lbs. loaded. Trailer is dual axle w/ 4 wheel brakes.

Often when I attempt to brake, the truck ABS system seems to engage and the rig BUCKS uncontrollably. Happens more often if going down hill. Happens at low speeds as well as at freeway speeds (like when some JERK pulls 20' in front on me on freeway and then brakes hard...like I can stop on a dime witha 60' rig!) This is REALLY dangerous (I almost lose control, luckily haven't hit anyone) and is wrecking my poor horses who get tossed around.

I've asked my Ford dealer and they feigned ignorance. Trailer dealer said they had a similar problem with a 2002 F 350 and when they got a 2007 F 350 the problem went away. Since I can't afford another $50K truck right now, does anyone know a solution to this problem?

Truck pulls my 2H w/ tack room trailer (7K lbs. loaded) with no problem braking.