From what I understand, Jackson used to fill in at safety for Bob Sanders (when he was injured). I also thought Jackson was originally a FS upon getting drafted, but was moved to corner in Indy's cover 2 scheme (at that time). Given his measurables and play history, I see no reason why he couldn't play FS.
As noted above, I too am concerned with his injuries as of late. If he could pass our physical exam and will accept a reasonable (small) contract, I think he has a lot of talent worth taking a gamble on.