It goe away for the most part when i hit the brakes or atleast decreases atleast 50%.
I think you have two issues to deal with, first you need to get it driving straight, but then I think you will have a pull to the left under braking.
First let's get the alignment squared away with slightly lower caster on the left and matching camber or just a tenth of a degree or so higher on the left. also make sure your tire pressures are even before it's aligned.
If the alignment shop can't adjust it as specified you need to ask why. There are "knock-outs" on the upper control arm mounts that need to be removed to allow camber/caster adjustment, have they been removed?
If your truck still doesn't drive straight swap your tires from side to side (pay attention to wind direction and drive on a good road with no ruts or major tilt to one side). This will rule out a radial pull. if your tires are directional you shouldn't leave them like this, they will wear out faster and won't get proper traction in wet conditions!
If you get it driving straight and find that it pulls under braking try the parking brake test that I suggested earlier, pull and hold the release handle and apply the brake lightly, if that causes a change (pulls or stops pulling) it means that one side of the rear brakes is grabbing sooner/harder than the other. This may be fixable by tightening the adjuster on the weak side or loosening the one on the strong side, but you should inspect the rear brakes to rule out leaking seals or other mechanical issues.
If it stops straight using the parking brake but pulls when using the regular brake pedal then you have one front brake grabbing harder than the other or something is shifting in the steering/suspension when you hit the brakes.
I have also seen tires cause a pull under braking, I don't really understand why. Probably similar to a radial pull where all of the radials inside the tire are not the same diameter.
Without putting my assometer in the driver's seat I can't offer much more help.