Might be the same as 1988 trip reported elsewhere. These dates seem awfully "flexible". Either that or he made a lot of trips to Africa in his 20s.
But I don't think I'd put much stock in that article. It's just dripping with praise for Obama and his family, and takes pot shots at the British, the other tribes of Kenya, and just about everyone not a Luo tribe member.
However, one interesting section says:
Dorsila, who spoke only Luo, listened nonetheless as Obama and Auma’s “Granny”, the same step-grandmother, now in her eighties, who Senator Obama and his wife and children met with in the summer of 2006, shared the oral history of their family.
This implies that the conversation with Granny Sarah was in some language other than Luo, which means it probably was either in English, or Swahili with someone interpreting for The Messiah, both official languages of Kenya, and with most people speaking at least Swahili in addition to their tribal language. In this case Luo, aka Dholuo (as Swahili is Kiswahili, in the Swahili language). That kind of pokes a hole in the theory that Grandma Sarah doesn't speak Swahili/Kiswahili, which some have used to discredit this "affidavit".