Look at #6 in upper left. That's the inner driver, it's made up of a tripod bearing & a 'cup' where the bearing slides in.

The tripod bearing is a three-legged thing that's attached to the end of the shaft. You have to line it up so it slides into the grooves in the 'cup' part. If you have trouble with it, take off the big end of the rubber boot so you can see what you're doing. Be careful not to get any dirt in there, & it's a real mess to handle with all the grease in there.

Also, be careful that any of the 3 bearings don't come off from the tripod. There's needle bearings in there, they can be a real handful to reassemble.