More info that might help someone trying to repair a worn axle.
According to RockAuto
A '51 1/2 ton pickup takes a National R1502E rear axle bearing which has a bore of 1.877", OD of 2.782", and width of 0.875". Since this is a roller bearing w/o inner race that would mean the axle would be slightly less than 1.877" in diameter at the bearing surface.
A '51 BelAir takes a National R1500EL rear axle bearing which is also a roller bearing without inner race. OD is listed at 2.4062", Width of 0.75" (no bore diameter listed, but This lists bore dia of 1.580"). Another bearing for that application Timken R1500EL is a roller bearing with both inner and outer races. OD and width are same as the National, and bore is 1.580", so that tells us that the axle diameter is slightly less than 1.580 at the bearing surface.
So it appears that car and truck axles are not interchangeable.
The link I listed may help find an inner and outer race bearing that could be adapted with some slight turning of the axle shaft.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.