The reason is most likely a problem of warpage in the front rotor disc and/or disc drum in the back. If your tires are out of roundness, it can also be the cause.
Could be a vacuum leak
Check for warped front rotors. under slight brake application it may not shake. But under hard braking such as getting off an offramp the truck will shake and the brake pedal may pulsate slightly. Chances are real good this is your problem.
Many things could cause an engine to shake. One, for example, would be low coolant or oil levels.
You have a warped rotor. Brake pads need replacing and the rotors turned or replaced.
Most definitely. Any miss will cause the engine to shake. A vacuum leak will also cause this.
If it only happens when you are braking I would suspect warped brake rotors. Other reasons the front end could shake would include unbalanced tires or the vehicle needing realignment. Whatever the case may be; driving a car in this condition is unsafe and you should repair it or have it repaired before you drive it again.
what would cause a rubbing noise on drivers side front wheel especially when braking ( I replaced rotor and wheel bearings are good) ?
Yes, it can.
To the FRONT of the vehicle
Normally caused by worn steering or suspension components.
Who checked them and how were they checked? Pulsing ONLY when you use the brakes is just about always the rotors. You might want to check for a bent wheel, but that would be an almost constant vibration.
if it is automatic then this would be caused by shift down is not working in transmission