to campaign vigorously for anyone in November 2004?
You cannot campaign for anyone while on active duty.

He started basic in May or June of 2002.
He enlisted for Ranger Regiment, which is typically a 3yr 17wk or 4yr 17wk contract.
(The 17 weeks is just for basic and airborne school, ensuring you get to RIP with 3 or 4 full years left.)
To be out of the military, he would have needed to be on a 2 year contract, (unlikely for a Ranger contract).
Even if that were the case, Tillman deployed in April 2004 to OEF. Stop-loss for that rotation would have prevented him from getting out of the Army before November 2004.