Well Johnhef, you were absolutely right! Dealer removed driveshaft, noted that the u-joint was binding, and replaced entire driveshaft (including center carrier bearing).

Car was at the tail end of the warranty, so it was covered.

No more vibration - it is SO nice to have a problem that has dragged on (literally for several years) completely fixed!

Thanks Johnhef for the great diag!

__________________
2001 e320 wagon
2004 F350
2006 Crown Vic