GE washer GTWN2800D0WW agitator slow and moves basket

[Cattress]

Instead of the agitator turning back and forth short amounts rapidly during the wash cycle, it slowly turns left-right, moving the basket with it. I would compare the sound of a proper wash cycle to be ch-ch-ch-ch, where as my washer says "swish...swish". It usually spins and rinses fine, but clothes are not coming out very clean. When I manually turn the base of the agitator, when the washer is empty, the agitator turns about an inch or so alone, then it "catches" the basket and moves the basket, both directions. I'm hoping I can diagnose exactly what needs to be replaced and order it before I begin to wrestle that nut that I have to get a spanner wrench for, and then the belt, since those sound like the hardest parts. Any idea what part needs replacement based on that?