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.
