Car vibration in 4th and 5th gear?

hi i have a rover 45 diesel 2000 reg i have just had the cam belts and and a new clutch fitted, and the car seems fine apart from, in 4th gear between 2000 and 2500 rmp the car vibrates and its the same in 5th gear but worse, its fine up to 2000rpm but when i put my foot down hard i get bad vibration from the car.it feels like i am driving over a rough road it only last for a few seconds then after it passes 2500 rpm it seem fine. i do a lot of driving so it getting to be a bit of a pain.