Severe vibration 1800 to 2200 rpm

The fact that it does it at the same rpm in each gear suggests that the problem lies forward of the gearbox or with the gearbox itself, as the rotation speed of all parts after the gearbox is matched to road speed, not engine speed.

Was the gearbox rebuild and new clutch a 'professional' job or DIY? Has it been like this ever since the work was done or was it ok to start with?

It was done through a local garage who have done all of my work for a number of years. The vibration was definitely not there before the clutch and gearbox was done. I think you have a point as it is not there in 1st or 2nd at all. I will have to check what clutch he fitted but he is away until Wednesday. I rang a local propshaft balancing company and they suspected uv joint or prop out of balance. I will wait until next week and see what the garage proposes. Thank you for all of your help, it is much appreciated I will keep you informed of my progress.

I'd be very surprised if its a prop. or UJ to be honest. They would cause vibration at a certain road speed, regardless of what gear you are in.

Did the garage drop the prop shaft in the pit when they were changing the clutch. A mate of mine built a special with shortened prop shaft some years ago and had to have it balanced to get rid of the vibration.

how to solve this problem? In my opinion it is vibrating motor at 1800-2000 RPM in second gear

Damn, I am having the same problem here 😬.
From third to fifth gear....extreme vibrations. Also after clutch change 🙁
Anyone....any conclusion?