I had this same thing happen to me in a 91 except it stuck in 5th and would not come out... sucked. I burned the clutch in 5th just getting it to move but then it popped out and would only go 1-3-5. Turned out to be the pile of **** B&M shifter I had in there.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.