I just replaced a torque converter that was going out and it had a vibration at about 2,000 RPM regardless of the gear it was in. Didn't affect how the engine performed, just started getting a shake to it.
I suppose they can fail in a number of ways, but when they go completely you may toast the rest of your perfectly good tranny (who cares aboutt the miles) with pieces of torque converter. A simple rebuild on the tranny and a new torque converter now and you're good to go - you sure you want to wait????