Yesterday I changed front and rear flex disc on my 1997 C280, and when i got out on 4th gear at like 1900rpm the car started vibrating but it was not happening on all the other gears and only in 4 at 1900rpm. Please let me know if there was special way of puting them on the car or just did something wrong. Thank You.

 

Are all the bolts tightened correctly? Did you look for the marks on the shaft (I do not know if this is the proper word, but it is the thing that connects the transmission with the rear axle)? They should be respected to avoid vibrations.