Seem like it's the back brake calipers and I'm getting a bad shutter when I make right and left turns

Whenever I turn the truck on, when I start driving it, for the first few stops the brake catches as if i were slipping on ice. However if I pump the brake, its fine for the rest of the drive. Also, the ABS turns on whenever I reach 10-15mph after I start driving. A few other things that may correlate include the speedometer not working and my 4x4 not engaging, thought not positive. Ive replaced the calipers, pads, speed sensors, and 4x4 actuators. My dad seems to think it may be the abs module, but wanted another opinion. Thanks in advance

I changed the calipers and new brake booster and new master cylinder and bled the brakes still hard to brake

Put new brake pads ,master cylinder,caliper. Brake peddle goes to the floor