The rear differential is not real complicated inside and new and used parts are readily available to fix it. Do you hear clunking or grinding when you turn the rear wheels by hand? If so, time to split it apart and see what broke. You can download the 2000 series 2011 and after service manual here that describes removing and disassembling the rear differential.
https://www.ccpartsnmore.com/cub-cadet-service-manuals