OD shifter position ALLOWS 4th gear...but 4th gear actually engages only at 80 kmh normally...but well, it is possible . TCC, on the other hand, does apply in 3rd gear, the top gear in D, this based on my '93 with the 3.8 YOu can detect this by noticing a small jolt and slight lowering of rpm while car is driven in D at constant speed of very moderate acceleration over 40mph/70kmh .
What you describe seems to be caused by lack of hydraulic presssure.....and over 70 mph in 4th, the engine runs the transmission pump fast enough to generate adequate pressure, which you get at a lower speed in 3rd gear since engine runs faster.
You have the vacuum transmission modulator in '96...it controls the pressure in the transmission.
www.avigex.ca/xport/modulator97.jpg
www.avigex.ca/xport/modulator.jpg
If you unplug the vacuum hose to the modulator, this mimics full throttle and transmission will operate at maximum pressure. Try this for a test and report results...MAybe the modulator irself is faulty too....that would be an easy fix.