The bearing is going to be compressed just a bit when driven into the axle housing (and the axle housing is going to expand just a bit as well.) That might be enough to decrease the effective diameter of the rollers where they slide on the axle, making it hard to slide the axle in. It's also possible that the axle housing is bent a bit (if the truck had ever been overloaded) causing the axle to bind when it goes in.

I had a '72 K10 that the previous owner had overloaded with a humongous slide-in camper and bent the axle housing. It had a positraction differential and wound up breaking the "petals" off the clutch disks and throwing them into the pinion bearings. That entire axle was trashed.


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.