After reading the responses now I have to do it. Ashtray: just want to make sure that we're referring to the same dust cap. It's shown in the attached pic, forming the walls of the cup at the end of the shaft.

 

Just wanted to follow-up with some post-rebuild notes.
- A local driveline shop pulled out the centering ball for $5 here in Denver. They do a lot of Disco shaft rebuilds and had the rubber boot in stock as well.
- The guy at the shop mentioned that the condensation drain hose for the air conditioning system drops water directly onto the front shaft. He recommended that I use zip ties to redirect it away.
- I had a bit of trouble re-assembling the double u-joint end. The new boot was stiff, and getting the u-joints to be centered in the yokes to insert the last two caps was a bit tough. It helped to assemble that end without inserting the retaining clips until the very end. The extra movement was enough to get things centered.