one thing I have seen twice in the last two years or so on two different mustangs. If you get under the car, look at the side of the tranny about 1/2 way up and 1/2 way back on the driver's side. there is a round-head bolt with a torx opening. This bolt is the actual pivot point inside the transmission for moving the shift mechanism around. It should be flush against the side of the tranny case. If it is backed out significantly, it needs to be tightened back up. But it is not so simple and you need two people. One underneath with a ratchet with the correct sized torx bit, and one in the car with the transmission in neutral moving the shifter back and forth. the person underneath has to carefully screw the bolt in using little force. If it binds, back off just a bit and get the person to try to shift between 1-2 or 2-3 or whatever. And as the shifter is moved, the bolt will go in without excessive force. Once it is all the way in, torque it down that last 1/16th turn or whatever.
No guarantee this is the problem, but it happened to ours. Started off not going into reverse, although 1-5 worked fine. We continued going to the strip, and occasionally driving it around town when we could avoid situations requiring reverse. Then it started getting balky at going into 1/2, although 3-4-5 were OK. I then noticed that if I first shifted into 5, and tried to shift into R (which would not happen) then I could get it into 1st every time. We took the thing apart thinking it was something major, but after removing the tailshaft and the bell housing, we discovered that the problem was just the loose pivot bolt and it has worked perfectly ever since...
We bought it used, and could only conclude that maybe someone wanted to change the tranny fluid (it had synthetic when we got it we discovered) and they might have initially thought that was the fill point and figured it out after a bit...
