Your 744 non turbo has the AW70L in it, its lockup transmission with the oversized Torque converter, when a torque converter failes it fails in such a way that the torque is no longer sent into the transmission making it possible to no longer go in any direction, i recenelty had one go out on me, the do some interesting things when they go out but they do not stop the transmission, think about it this way if it went out doing 80mph and it didnt want to go forward anymore what would happen? see my point. It sounds like the Gear Shifter has slipped and when you put it in gear your one gear out but then even if that was the case in reverse you could tech push the car in any direction. I would at this point pull the transmission and see if you have some damaged disc's or maybe your valve body is blocked and its holding it in gear.