First, I don't think AdComs will look at you in a negative light if you have taken 6 years' worth of undergrad coursework. I'll remind you that the average matriculant is 24 years old (so it is much more common than people realize).
As far as the formal vs informal post-bac, let me ask you a few questions first:
- What is your current cGPA/sGPA?
- What is your year-by-year breakdown?
- Could you give me a general idea of your ECs?
- Are you opposed to going the DO route?
A 3.3-3.4, though still a bit below average, would not be devastating if you had a steep upward trend over the last couple years. If you were to concentrate on upper-level science coursework over the next year and ace your classes, what would your new sGPA be (3.3?).
I agree that in your case a post-bac wouldn't be much more beneficial than simply taking classes at your alma mater. Have you considered an SMP, or a post-bac with linkage?