Buick Regal Lock-up Trans

1992 Buick Regal Gran Sport, less than 46K miles on odo (yes actual) - 3800 V-6 w/4 sp auto trans and lock-up torque converter. Check eng. Lght. On intermittent. Shop checked code, related to interrupted signal to trans lock-up sensor. Car has tach and have watched what happens. Regardless of check eng. Lght. Torque conv. Sometimes will lock up/sometimes won't. Trans shop says can damage/overheat torque convt. If driven on freeway whithout locking up, because converter is slipping. But in older cars, without lock-up, trans always slipped a bit anyways? I NEVER drive more than 55 m.P.H. (Yes, I'm an old, slow fart), which is about the speed lock-up takes place anyways. R.P.M. @ 55 with lockup is about 15-1600. Without lockup is about 1900-2000. Will I really damage trans or engine if driven without torque converter locked up? Or will I just suffer a bit in fuel mileage. (Which, by the way isn't great for a V-6, I get about 16 mpg in town and the best I've seen in hwy driving is 23 mpg. Should I be getting more with this engine?)