Anybody know anything about a Stock Model T?

Pinion is keyed to the end of the solid bar torque tube inner.
More likely is the babbit thrust washers in the rear axle. Normally just one side wears down, if left long enough or an EP oil is used they disintegrate and the crown wheel can move out of mesh with the pinion. Well, almost. Sounds like you've gone from almost to definitely!

 

I really fancied having a go until I read this thread...I've just re-read it all and I'm still lost!
Tell you what, sell the chassis and give me the body, I'll find an A frame to drop it on

 

What? My £10 T bucket? No chance!
I'm selling it with the sedan body, or keeping it to build a tourer.

Best thing about T driving - no matter which pedal you jump on, it'll slow you down...