But if it was a uj, diff, drive shaft etc wouldn't that affect al gears not just 4th

It seems that no-one else is coming up with a definitive answer, but making suggestions to you. It isn't hard to check the UJs, and it isn't beyond the realms of possibility that this could be the issue.

You don't need to take my advice, but it is easy to rule out the UJs with a quick look, you don't even need to jack up the car.
 
It seems that no-one else is coming up with a definitive answer, but making suggestions to you. It isn't hard to check the UJs, and it isn't beyond the realms of possibility that this could be the issue.

You don't need to take my advice, but it is easy to rule out the UJs with a quick look, you don't even need to jack up the car.
Always worth checking the UJ's and greasing them too.
 
But if it was a uj, diff, drive shaft etc wouldn't that affect al gears not just 4th

By fourth i take it you mean in torque lock. In torque lock the convertor is locked so the engine is direct coupled to the gearbox as a manual car would be. So any vibration will be more noticeable than out of torque lock when it can be absorbed by convertor slip.
 

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