1993 Torque converter shake, rattle, and roll

We've got a '93 Bonneville SSE, 3800, Auto 140,000 miles. The torque converter recently started to shake and rattle the whole car when it is in the lock up mode. It doesn't do it all the time but in high torque situations, like 60 mph on interstate coming to a small grade it will vibrate like crazy til you touch the brake pedal and unlock the converter. I put in a can of trans additive and it helped alot. I ran it a week or so and then had the trans fluid flushed. It is much better but my wife drives it and I can't leave it like this. She loves this car and doesn't want to get rid of it but I can't see putting high dollars into fixing this. Anyone know another fix short of new torque converters and rebuilt trannies?