Craftsman 536270270 has no speed in grass or any incline?

Hello, it starts right up, blades will engage and will be fine on a level surface. If there’s any incline whatsoever it slows to a crawl (even on concrete). I’m going to try replacing the transaxle belt as suggested by the previous owner. Anything else I should look at? Drive belt looks OK but since I have to remove it anyways I’ll replace it too. Thanks!