No idea what it is, but something has aggravated the problem and its been there ever since I had the truck and its become more than a nuisance at this point. Now it just seems dangerous.

If you are running disc brakes the piston could be cocked in the caliper. If your running drum brakes, the shoes could be out of adjustment, closer to the drum causing it to grab first and harder the more you stomp on the pedal.

Often this can be a suspension problem, worn bushings, loose tie rod end, Track bar starting to get worn, Radius (trailing) arm bushings, C bushings etc etc.As well as alignment being off. Drum brakes can make it pull as well and or a hydraulic issue with the brake system itself. check everything over really good and make sure your drums are adjusted and you have even temperatures at the brakes front/rear 60/40 and the suspension is not worn out.

I agree with 69 Bronco. If your still drum all around check the adjustment of all the brakes. The rears can cause the pulling as easily as the front.

I remember before my dad had the discs installed on the front of his Bronco that thing would pull to either side under hard braking. We rebuilt each wheel cylinder and adjusted each brake very carefully. Even after that under a panic stop you didn't know which way it was going to pull. Installed the disc conversion and it brakes straight as a string.