basically if i put my foot down in 5th gear at around 40-50 the car judders really really bad like there is an earthquake in the car or the feeling of a bent driveshaft, also struggles at 35 mph as well but not as bad. anyone got any ideas at whats going on?

i know it isnt a bad driveshaft but feels like it. its a tdci sport on 26k, you can see it in my signature.

ive got an idea what it is but if ike your thoughts as well.