Thank you for your response! We tore apart the rear diff and you are 100% correct. The splines inside the rear diff and drive shaft/coupler piece were stripped. Poor design... no zerts to grease it and it was pretty badly rusted out. This issue began by simply pulling out of the driveway after it had been sitting a few days. The use before that was also mellow, level ground, nothing difficult or torquey. Just rusty and hit it's limit.
FYI for anyone with sinar issues, ee found good prices for the HiSUN Sector 450 parts via telephone from Yarborough Equipment in Strafford MO.
camojay's post in Hisun Sector 450 (2021) - help diagnose rear diff issue (short video included) was marked as the answer