When I speed up and let off the accelerator, my truck makes a quick shaking, I think its coming from the back. It does the same thing when I'm slowing down with my foot on the brake it makes a quick shake. I have no idea what this could be.

 

Soooo... it shakes all the time?
Any more information you can provide?

 

check your drive shaft u-joints. park on level ground, block wheels both ways, trans in neutral, engine off, e-brake off, and all tension off the drive line. try to wiggle the joints. there should be zero movement. any movement at all is worn out.

 

do you have a lift? Because your U-bolts that hold the blocks to the axle could be loose. Happened to me in my 1996 Ram. But that was when I was accelerating. But it got better when I was at about 50 MPH.

Also, it could be your front ball joints. THat is if you are wrong about where the shaking is coming from.

And yes, what DHvaughn said. Check your u-joints.

 

WAIT!!!!

It could be that your brake shoes in the rear need replacing. Also, if the hardware (Springs) in the back, are really rusty...it cam do things like that. Also, it could be your rotors being warped, or your pads need to be replaced.

Also, check your brake fluid.

 

Ok, thanks for all the suggestions! I will check the U-joints first because that seems to be the easiest. If thats where the problem is, I'll replace that and go from there.