He missed most of the 2002 and 2004 seasons, all of 2005, and parts of other seasons, including this past year at Double-A West Tennessee, where he underwent surgery to repair a hip-related injury suffered in late July.
Six-plus years after being drafted, Jackson has decided to test the minor league free agent waters, the Cubs say.
"He wanted to see what was new for him out there," Cubs Farm Director Oneri Fleita said. "He was coming off another surgery, one to repair his hamstring. He's had a lot of injuries, so that's his right. He earned it."
Their contract status is unknown and an official list of the players who opted for free agency was not immediately available.
Fleita said he couldn't remember which players re-signed with the organization through next year and which didn't.
"I don't really have that [list]," Fleita said. "The guys who wanted to come back came back and the ones who didn't didn't. That's their right and they've earned that right. It doesn't mean that we wouldn't sign them back."
Beginning Sunday, teams can sign players who became minor league free agents as of Oct. 15.