'91 Taurus AT wont shift

Hi, this is only my first post to these boards so hopefully I won't violate any rules.

I have a '91 Taurus with an automatic tranny that won't shift by itself.
It will drive in any selected gear (R,D,2,1) but it won't shift out of the low gear. Has plenty of fluid but that's about the only thing I would know to check because I've never done any work on transmissions. (I only know that they're expensive to fix)

Could it possibly be a logic module problem, speed sensor, clogged screen or are transmissions just toast when this happens? Also, I have a generic handheld module that will spit out Ford error codes but havn't got around to hooking it up yet. Would that reveal any problems with the transmission.

Really need some help here so hopefully someone will have some experience with this type of problem

Thanks
Mikey_P