94 AXOD 2-3rd upshift trouble Please help

O.K. Here goes nothin. Transmission always shifted beautifully until two days ago....I had a couple of rough shifts from 1st to second......seemed a little peculiar...so I decided to swap the fluid and the filter......on the way home from the parts store(figures)the car suddenly decides not to shift from 2nd to 3rd or into overdrive. It will take off in 1st gear and shift smoothly to 2nd under gentle acceleration. If I hammer the gas pedal :) it accelerates nice and tight....it just wont upshift to 3rd at all. The condition of the used tranny oil...nice and deep red color...no real burnt smell....pan magnet lightly coated with fine fine metal...no chunks. The car will operate in (1)(2) and (D). It also has no trouble shifting down from 2nd to 1st when at a stop. If anyone has any detailed info...I would greatly appreciate it I love this car...but would like and inexpensive fix :) Thanks