I was working on a 1996 chevrolet 1500 suburban and extended cab truck in which the rear brakes which are 10 in. brakes aren't working right

I was working on a 1996 1500 chevy suburban and a 1996 extended cab
chevy truck (both have abs systems) in which i replaced the complete rear
braking system including the drums, but still the two are having the brake
peddle act up as if it were abs. I was also wondering if it might have been
something with the the booster or other braking components. I know its in
the rear because the fronts were changed and grab their own rotors. The
Rear shakes because of the brakes. When i don't push the brakes the
vehicles run fine. anybody have answers