When I make a sudden brake then step on the gas it feels like the rear shifts. I'm not sure if it's the bed, drive shaft, or a bad abs system.
My car has an automatic transmission.
This may very well be a bad U-joint in the drive shaft. When these have too much slack or play in them, this can cause the same feeling you are describing. As you accelerate and more torque is applied to the drive train, the U-joints are responsible for transferring this power from the transmission to the rear end differential. When these have too much play or slack in them, this can cause the slight movement you describe. I would recommend having an expert from YourMechanic come to your home to diagnose this issue firsthand and service your U-joint as necessary.
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