Any good A&P can do this. Best bet is to find a shop that specializes in major structural repair - they know how to do it right. You pay for what you get.
If this is the proposed AD, then hold off a bit.
The backseat of most cherokees is just sitting on the spar. You can remove the seats with only a bit of trouble, making 3 sides of the spar easily accessible. The bottom of the spar, well, that's where the little holes on the bottom of the wing and fuselage come into play. Next question - what type of inspection of the spar? Visual only? Something more comprehensive? In Florida, I'd worry about corrosion, which is why you're probably looking for an inspection.