I would say your clutch, since the problem seems to only occur uphill. The thing is, you say it still engages in the lower gears. I'm baffled. I would say the gear itself, but you say it takes on flat roads... Try doing a rev-match when you shift, and report back.

Most vehicles I've had wont take a decent hill in top gear. My '94 Golf WAS capable of taking hills in 5th but it had a very torquey engine and 5th gear put the car at the speed limit at it's rated peak torque RPM. Since many automatics are 4-speeds, most will shift to 3rd automatically upon encountering the strain up climbing a steep hill. In this respect, I doubt that your '91 T-bird is different that my old '95 Saturn, '98 Saturn, '89 Aerostar, '91 Regal, '93 Taurus, '95 Cirrus, or '94 Legacy; all 4-speed slushboxes. The only automatics I've had that didn't downshift on hills were all three-speeds ('87 Corolla, '87 Golf, '89 Golf, '94 Corsica). As for it being cold, my '94 Legacy won't even think of hitting 4th until the temperature gauge starts to move since the temperatures dipped below freezing at night. Lower gears plus unlocked torque converter plus hight RPM equals quicker heat generation. The car is likely trying to reach normal operating temperature ASAP. Turn the heat on high and feel the air coming through the vents. If it's cold or lukewarm, there is your issue. If it's blistering hot and you're still shifting oddly, get back to us.

It sounds like the transmission is trying to downshift into 3rd gear and is slipping out of gear instead. I don't like it. My first inclination would be to flush the transmission filter/fluid and hope for the best. If that doesn't fix it, then you're probably looking at either A) Living with it or B) Having the transmission rebuilt. Good luck.