It'd be an interesting one. I can see Jackson bamboozling Witherspoon on the outside, but Witherspoon doing well at mid-to-close range. If Witherspoon boxers off the jab and focuses on throwing well-placed quality punches (to contrast with Jackson) I think he could win a contentious but well deserved decision. If, however, he fights like he did against Bruno, with bursts of high quantity but poor quality punches, he's playing into Jackson's hands. He'd get confused, outworked and frustrated, and probably go down on points in a similarly close decision. I think it's most likely that a prime and focused Witherspoon will go for the former option and win a unanimous decision (something like 8-4 or 9-3). This would be an entertaining fight, I think, because the way they match up it's a recipe for a lot of sustained and colourful action.
Because Jackson was a skinny heavyweight without a big punch who was stopped by some guys who did not hit as hard as Witherspoon. Tommy Jackson was not that good. He was just a contender type. If Witherspoon shows up lazy, Jackson has a chance. Otherwise its an easy night for Timmy.
I tend to think that Jackson is a guy who only got as far as he did because he was unorthodox and people often took him lightly. The fight is esentialy Witherspoons to loose. If he treats Jackson as a serious obstacle then the chances of Jackson wining become verry slim.