my sport 1100 had the same problem for years... vibration only in 5th gear and u joint loose on the shaft. Yes it's a problem and yes change it soon. Mine could only be tightened with the cap screws done up to a zillion footpounds. When I got the new one the difference was immense, before the u joint would slip on and off real easy, with the new one I had to have the orientation exactly right before it would slide on and then I only had to torque the capscrews up to the value and it was tight. All the vibration stopped and it I was able to use 5th.
A wiggley u joint is not good for either the gearbox or bevelbox.