I do the same thing every year. As others have mentioned it will be $75 for over 50lbs and $75 if over the 62 linear inches. I've found it best to do multiple smaller coolers (right at 48-50lbs), but it can get difficult with the number of coolers when I'm by myself at the airport. I put the backstraps/tenderloins in my soft-sided cooler as a carry-on (not weight restricted). Never had any issues with freeze thaw.
I'm flying out to BZN from BWI in Sept as well. I must say, SW has been really crappy the last 18 months, but never had a problem with them before this time period.
As far as the rental car, instead of a truck why not look to get a large SUV? That will provide the protection you need and some extra overhead space.
For rentals, costco or autoslash.com.