For some reason I have yet to see Uber Diablo. Apparently, he can spawn randomly in any game difficulty without a Stone of Jordan being sold. That hasn't happened to me, and I don't have a Stone of Jordan to sell either...
I did, however, do 4 Uber Tristram runs with a personal friend of mine.
Good luck with getting Uber Diablo.
I have a level 89 fire sorceress, but, I don't do the actual Ubers in Tristram. I help my friend get all the keys, and with the keys, get all the organs, then I use my barbarian to shout on my friend's paladin, and he does all the work in Tristram while I just wait and shout when he needs it.