If the rotors are warped it can cause a shake when you hit the brakes.
Most probably
Warped brake rotor.Warped brake rotor.
Run the truck backwards to about 10 Mph, hit you brakes very hard. This will adjust your brakes in general.
You don't give the make or model, but on most older trucks if it has disc brakes, they are self adjusting. If you have rear drum brakes, most of those, you put the truck in reverse, go back till you reach say 5 mph, then hit the brakes. This turns a self adjusting start screw. Back up and hit the braks agin and again till the brakes work better. If it still don't work you may just have to have need pads and/or shoes. HTH, Chuck
One of the shocks on that side is probably bad.
It is a command, likely followed by an exclamation point. "Hit the brakes!"
replace the front brakes and rotors
That means person inside car is either too heavy or something like a toy is caught in engine
You have a serious problem. I may be easy and cheap to repair up to complete engine replacement. It will need to be diagnosed by a knowledgeable person.
have your brakes checked
Seems reasonable. Kinda makes sense if you think about it. If it seems excessive I would ask your local tire/suspension shop to check your shock absorbers.
Spring brakes are intended to be used solely as parking brakes. If you're in a desperate situation, you can use them as an emergency brake, but if you're grossing out at 80k and going down Vale Pass, all you're going to accomplish is to burn up your brakes and possibly cause your truck to catch fire... best to just hit the runaway ramp at that point.