Do you also get a slight vibration in 4th and 5th around 40 MPH? under light throttle? A dragging torque converter clutch will cause this. Do a search for " Sprinter RSN " ( Rumble Strip Noise. ) This can also be called " Torsional Vibration "

My 97 SL320 and 97 C280 both have this vibration with the C being worse ( lighter car ). It feels like a standard shift that is in too high a gear and the throttle is lightly pressed..

Also look at Sonnax transmission products, they have kits to fix various valve body issues. Some need special reamers so pick and chose what you buy. There is a torque converter buffer sleeve and spring kit ( or similar name ) that applies to "late" trans , not sure if 2000 is late enough.