This question comes up a lot, so I thought I’d start a thread that may hopefully help out. A Johnny Joint is either come as a fixed housing Weld on style like on an upper axle mount or they come as the type with the threaded section for use with an adjustable arm. The Johnny joint is a very large, tight joint, but the ball assembly inside allows smooth pivoting and less binding as the axle moves. They bind less than a polyurethane bushing are are affordable to rebuild.
Here’s a picture of a Johnny Joint’s internals. Basically, the order of parts that goes in the metal housing is WASHER, BUSHING, BALL ASSY, BUSHING, WASHER, SNAP RING. You can see that the housing has a snap ring groove and a hole for a grease zerk.