I would try rotating the spare in place of each of the 4 tires to see if there is any effect on the vibration. You could have a bad belt in a tire and just not know it...

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280