On the earlier 4x4s, they had a closed knucke that used king pins and a ball shaped enclosure around the outboard U-joint. The ball is 2 peice, with a felt wiper type seal between the 2 parts. There is a plug on it to allow for addition of gear lube for the joints.
Open knuckel is what was used on later trucks, it's still used on the F250/350 4x4 ans the Jeep Wrangler. Basicaly, there is no enclosure around the U-joint, so it is "open". These axles are usually equipped with ball joints in stead of kingpins, with the later 1 ton GM/some Dodges being the exeption. These have a much tighter turning angle than the older closed type.
If you're looking for a stonger axle, rather than 100% originality, go to the open type. They have larger/stronger spindle mounting, and are less prone to breakage. GM axles up to '77 are the best, though some 3/4 tons use the beefier Dana 44 axle up to '87. Any 1 ton front axle up to '91 will be very strong, too. This would be an open kingpin style Dana 60.
