If your vehicle is showing rear end squats down heavily when accelerating, here are the most likely causes and the parts you'll need to fix each one.
Shock absorbers control the speed of weight transfer. When they wear out and lose their dampening ability, the rear end will rapidly compress or 'squat' under acceleration.
Springs support the actual weight of the vehicle. Over time, heavy use, or towing, they lose their tension and allow the vehicle to squat easily under load.
If standard light-duty shocks or springs were recently installed on a vehicle used for towing or heavy loads, they will not provide enough resistance against squatting.
On vehicles equipped with air ride, a failing ride height sensor or leaking air suspension solenoid can leave the rear end under-supported and prone to squatting.
Degraded rear control arm or leaf spring shackle bushings allow the axle to twist under acceleration (axle wrap), exaggerating the squatting motion.