Mark,
The 15000# axle was the GM Corporate axle that had vacuum shift. The 17000# axle was the Eaton axle and they had electric shift.
I found a 6.14/8.36 Eaton rear center section for my 1957 10500 Fire Engine. With my 1:1 5th gear I run in the mid 60's all day long, I'd be underpowered to turn any higher gears.
Mike B
