Old thread but figured I'd share my experience incase someone comes across. I rented the ball joint press from Advance Auto. It's perfect for U-joints. I tried using a large c-clamp and it wouldn't budge the caps out of my stock u-joints that were on the truck for 160k miles.
Put the new ones on and pressed them back. Used some anti seize on the caps to help guide them back. One thing I read was to make sure the openings on the driveshaft were super clean, so I used my dremel attachment and cleaned it all up. Had a little difficulty with some of the clips that were supplied, it was a balance of not pushing one side in too far than the other. I found that the first side I pressed in the c-clip was loose until I started pressing in the other side.
I did use kroil penetrating oil on all the caps and let it sit for a couple hours.