Sizing Transistors for 2.2hp universal motor controller
I'm looking at adding a flyback diode for the motor


Very sensible and is likely the reason why your MOSFETs immediately died. With the inductance of the motor and up to 15 amps flowing as soon as the MOSFET turned off the energy stored in the motor's inductance would produce a massive back-emf (possibly up to thousands of volts) and smoked your MOSFETs.

So you cure that problem and then you might be faced with: -


Your MOSFET driver not being adequate to drive the gate capacitance of both MOSFETs wired in parallel. Please provide a link.
Your circuit board (or dread to think breadboard) layout may not be a healthy design that prevents other nasty things happening.
A 10 volt gate drive as your schematic implies is adequate for the MOSFETs but you say in your text that it is "PWM to gate (0-5vdc)" and 5 volts is far too low for these MOSFETs.
Your motor stall/start current could be in the realm of 60 amps and with the on-resistance being 0.03 ohms (both MOSFETs together), this would be a power dissipation of 3600 x 0.03 watts  = 108 watts and is probably OK.