Most likely trans rebuild time. The 5R55 transmissions in the 05-10 cars are notorious for having issues later on in life especially if the fluid is not regularly changed.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.