This tip comes from my friend Pat Fagen, owner of Axle Exchange/FastShafts in Des Moines.
Fagen says some sort of hydraulic or mechanical press must be used when repairing non-greasable universal joints. When assembling greasable u-joints, the kind with a grease zerk on the "cross" of the u-joint, it's fine to use a vise, a hammer and a socket of the proper size to compress the caps of the u-joint to fit inside their yoke(s).
But Fagen says the caps in non-greasable u-joints have a stiff neoprene disk that keeps the needle bearings in place as well as prevents the end of the cross from contacting the inside of the cap. He says you'll drive yourself nuts, or damage the u-joint, if you try to use a hammer to pound the cap(s) into place because the neoprene "bounces." He says to use a press to apply steady force to compress the neoprene disk and for proper assembly.
If you ever have opportunity to watch a professional driveshaft/u-joint installer rebuild a u-joint, do it. If you have to PAY for the opportunity, do it. Those guys know tricks that make rebuilding u-joints almost easy. It would take me 1,000 words to try and explain what they do in 30 seconds with a quick tap and a solid rap with a hammer, once they have things prepped and aligned. It's fun, and humbling, to watch a pro in action.