I have this problem to.
It only happens in 5th gear, and only on really flat straight roads, and only between 4pm to 5000 rpm.
The only fault I can find is that there is some play where the rear U joint mounts onto the driveshaft.
I bought another 2nd hand driveshaft, only to find that it was worse than my original.
I've looked really closely at the rear U joint, but I still cannot find out how it is attached to the driveshaft. I know that there are some splines in there somewhere. Other than carefully cutting my spare driveshaft assembly apart to see how it fits together I don't really know how to proceed.
Meanwhile, just go faster, that seems to work, or change down to 4th.
DD