It may be just a Saginaw, but it has three shifter shafts, a selecter shaft, a dozen detent balls and springs, four shift levers and the usual three shift forks. You obviously have a '66 or newer Corvair or at least the trans is.
That said, I don't think you would get "the body lifted like a brake torque" if the trans was locked in two gears.
This may sound wierd, but I'd be inclined to take a good look at the rear wheel bearing hubs.
