Gearbox locked in third..

[66STNG]

Hi guys looking for some help.
Ok,
BTR 4 speed auto out of a 2002 AU in my mustang.
Now, it shifts well into reverse, and will spin the wheels in reverse.
However put it in drive and it appears to be locked in third, possibly limp home mode.however I can’t get any codes up for the tranny, or any solenoid fault codes.
Any ideas or fault finding for me to look at.
Cheers guy