Speaking of whining, since having my DX 5-speed gearbox rebuilt by a very
reputable shop, I've noticed a rather distinct whine in 5th gear. (I cannot
recall if it whined before the rebuild).

This rather high-pitched whine (almost a whistle) manifests ONLY when 5th
gear is engaged, and the pitch varies with engine-road speed. It immediately
disappears when disengaging the clutch, and when letting off the throttle as
in engine braking. No such whine in any other gears.

Anyone know what this indicates?

I specifically requested the rebuilder replace the 4-5 slider; could it be
they installed a new Weddle 5th gear, which some report uses a slightly
coarser tooth design?
http://www.thesamba.com/vw/forum/viewtopic.php?t=640040&start=0