A lot of times with used washer a problem like yours occurs because the tub is not secured during transporation. As a result, the suspension springs may be sprung, or the machine may not be sitting correctly on its base.

You may also have a problem with the agitator cams, or "dogs" which cause the agitator to move when the motor turns. And, unfortunately, you may have a transmission problem.

The first thing I would do is try the machine in spin mode. If it spins ok, your transmission may be ok. I would then pull the agitator and check the dogs, and also take off the cabinet and check the suspension plate, pads and springs. Replacing the springs is a good idea since you cannot determine a problem on a visual inspection.

Here is a service manual that will show you how to get into the machine.

https://secured.whirlpoo...2007475cb/$FILE/L-55.pdf

Good solution