A833 Neutral Issue

If it does that with the side cover off and you can SEE the sliders are centered, then there is a problem IE the input pinion is stuck/ welded to the output shaft. Maybe someone ran it low/ without oil and the needle rollers in the drive pinion are AFU

I would try "a little more force" to see what you have before/ if tearing it down. Get a yoke/ driveshaft that fits and lock it some way. Get something to turn the input ?? a clutch disc?? and apply some goodly "arm" force. If it won't turn, big problem.. If it turns but hard, rough, big problem but you likely can get it apart.