First thing to check is the fluid if you haven't already. Make sure it's red in color without a burnt odor and at the proper level. On the 90-93 cars, overdrive is solenoid controlled by the computer, as is the converter lockup- each using it's own solenoid. It may be as simple as the solenoid is dead or gunked up. Does the converter lock and unlock?
The OD would blow up on them all the time. I had a A4ld that did it to me. When I took the trans appart the valve body was all gummed up with clutch material and the od band was broken.
