1998 Transport not shifting above 3rd gear, HELP

I have a 1998 Transport van. It ran low on transmission fluid and a leak was found and stopped. But since then there has been a 'service engine light' on and it won't shift above third gear (I sometimes wonder 2nd) on the highway. It runs at about 3,000 to 3,500 rpms at 55 mph. I don't have a code reader or know anything about them, but I am a decent 'shade tree mechanic'. Any help would be greatly appreciated.:banghead:

Buying a code reader is relatively inexpensive and easy to operate but I'm told AUtozone reads codes for free in the USA. Several codes will prevent shifting to 4th . NOrmally after condition is returned to normal the codes will vanish after a number of engine cycles but I do not have all relevant data for year 1998 and beyond so maybe they need to be reset with the scanner....and above 3000 rpm at 55mph is indeed no more than 3rd and no TCC lock.

Thank you, I also didn't know autozone possibly reads codes for free but I'll give it a shot.

Is there any chance that something could be damaged in the tranny since it ran low on fluid for an unknown period? My wife said the ses light was on for quite a while.