The gerotor is in the hydro pump.
Pull the rear wheel off and see if the hub has splines. The 2000 series service manual shows splines but it doesn't include the GTX series so they may have changed them. The parts look up shows it not to have splines but maybe it is wrong. Here is what the 3000 series splined hub & axle look like.
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If it is not splined and in good shape it sounds like something let loose in the rear and that doesn't sound good. I would call the shop you are taking it to and see if they ever split a Cub Cadet cast iron rear. I'm betting in my area that 99.999% of the shops wouldn't even know where to start. I'm thinking that is going to be a big labor job if they have to open it up. Can you post some pics of it or better yet put up a Youtube vid?