You should have an A4LD trans in a 1993.
It still used a vacuum modulator to aid mechanical shift points.
A4LD in your year had Torque converter lock up solenoid and a 3/4 shift solenoid, these are operated by the computer, and 3/4 solenoid could be the problem but you would usually get a flashing OD light if it was an electrical/computer issue.
And usually a problem with vacuum modulator would effect 1/2, 2/3, and 3/4 shifting, not just 3/4, but it is easy to check, and free, lol.
I would remove the vacuum modulator hose and check it for leaks/cracks, also transmission fluid, modulator can leak fluid into hose which throws off vacuum level at modulator.
High vacuum happens when throttle is closed, low vacuum when throttle is open, vacuum level helps trans to "know" when to up shift or down shift