1940 was the first year for the Hypoid design ring and pinion in 1/2 ton's.
1940 also went to a finer spline on the drive shaft.
The 40 will have two different length axles and the torque tube is mounted off center in the axle housing.
The 37 will have two equal length axles and the torque tube is centered in the axle housing.
You can swap the whole thing but you still need to overcome the spline count problem. This can be done if you go this way.


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