Not sure what model Mustang the OP has but, I have a 2004 cobra with std transmission. The owner manual states: all rear wheel drive vehicles place transmission in neutral, MAX speed 35 mph, MAX distance 50 miles. If you must exceed either speed or distance the drive shaft must be disconnected. Not worth the trouble to me. I purchased a 2003 Honda CRV as a toad. Place in neutral do not exceed 65 mph. It has a simple transmission routine to go thru before towing.
2016 Newmar Ventana
03 CRV toad