Truck pulls to the right when braking?

[Skunksmash]

Seems like I should know this but... what causes this? At highway speeds, my 87 pulls fairly hard to the right when you need to press the brake pedal down a bit harder than normal. Its enough that I feel like its trying to put me into the next lane.

So I'll be going 65-70 mph, and if I need to press down hard on the pedal for some reason, it starts trying to go into the right lane next to me. Anyone know what causes this? Its not nearly as noticeable when you are traveling at slower speeds, like just around town.