Your at the mileage where it might be your engine compression is uneven. Do a compression check. You might also want to update your linear valves and do a valve body kit from Sonax. If you think the vibration is from the trans remove the six torque converter bolts and start the engine and see if it smooths out. Go to the Sonax website and there is a bunch of info on the aw55-51sn. All the brickboard owners say stay away, but it is a great trans once the bugs are worked out. It is also easy to rebuild, just takes time. I have three 2001 cars and I have paid very little for them because of the trans problems that come into play from dirty fluid. Plenty of good improved parts for updates and a relearn takes maybe 55 minutes of driving. you just need a considerate mechanic to put car in relearn, and just drive the car and follow directions, it makes a world of difference in shift quality.