[LX5liter]

I think I would check out the tires first. Drive at the speed you are at when the tranny usually shifts except keep it in 1st, and if its not shuddering close to that speed, its probably not the wheels/tires. You will usually feel tire shake on the front end through the steering wheel. Some other things to check are the Driveshaft/u-joints, motor and tranny mounts, and possibly even the torque converter has lost a balance weight. I hope this helps.