1999 Chevy Suburban Rumble/vibration under load and slow sp

This suburban was given to us so I have little history on the vehicle. At low speeds, 30 to 40 mph for example you can feel a vibration under the front floor board, at least it feels that way. Took it to our mechanic and his first thought was it might be a driveshaft out of balance. Tested at another shop and it is right on specs. The mechanics next thought it might be the torque converter and said he had some aftermarket fluid he could add to the tranny that might help, but since it is more of an annoyance right now he suggested we wait to replace the converter when it was going bellyup.

Your opinion would be appreciated.