Started backfiring at about 40 MPH?

Yep, low fuel pressure will do it, please follow this guide and you can get a fuel pressure test kit at the local auto parts store for free or rent it for cheap. The fuel pressure should be

FUEL PRESSURE
1. Ignition "OFF" for 10 Seconds.
2. With ignition "ON," engine "OFF" and fuel pump running, pressure should be 380-420 kPa (55-61 psi).

I would change out the fuel filter as well if you have not done that with the tune up.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

and

https://www.2carpros.com/articles/how-to-change-a-fuel-filter

Instructions and location for your car are in the images below. Check out the images (below). Please let us know what you find. We are interested to see what it is. The fuel tank will need to be removed to change the pump out. Be sure the gas tank is empty before you start.

Good to hear, please use 2CarPros anytime we are here to help.