My truck randomly slams on the brakes...
 

...and it scares the shit out of me. It's a 1994 Chevy 1500, and the trucks applied the brakes by itself 3 different times now. The most recent was when I was driving about 25 MPH with my foot on the accelerator. It's done it all 3 times without my foot on the brake. I check the parking brake and it's definately been off.

Any ideas on how to fix it before it happens again and I get rear-ended?

You have anti-lock brakes? If so check your brake wiring sounds like you have intermittent shorts. If you don't have anti-lock there's nothing I know of that can produce a forceful slam on the brakes without you actually doing something.

The only thing I can think of is if the axle seals are leaking a little bit on the rear drum brakes, and the grease is mixing with the brake dust that comes off the shoes. Occasionally this will create a sticky paste which usually causes the rears to lock up on light braking, but I think it would be possible also for it to cause a random lockup if the rears are adjusted too tight.
Also if a hold down spring or return spring on one of the shoes has come off or is weak that could cause it.

For safety reasons, most if not all anti-lock systems can only reduce brake pressure. So that's probably not it. I don't know what it could be though.

the anti-lock/stability control system in most new cars(mine included) can put the brakes on to control skids or act as a traction control system, so it is not true that they can only reduce brake pressure. That is the true use of ABS, but in most new cars it has other uses. I would venture an educated guess though that in his 94 chevy truck that he doesnt have any complicated computers that could do that.

I bet it thinks a tire is slipping and that's activating the anti-slip. Take it to a dealer before you kill someone.

You would think a car without stability control would do that, wouldn't you? Trust me, leave it to GM to screw this up. His car, my car, I'm sure others do it.

Are you sure the brakes are locking up? Does this tend to happen after you have driven a while? It could be a bearing starting to freeze up, there is usually a noise associated with this but it would feel like the brakes being applied. Is it in the front or rear? A front bearing freezing up would cause a definate pull one way or another a back bearing may not. Just an idea

Possibly a tranny problem also.