-I would take out the wheel bearings, clean and repack them and then set play according to spec. If your bearings are shot or you have too much play you could definitely get some unpleasant symptoms at 55 mph.

-Something else worth checking: Your flex discs. Even though you said it was the wheel, the driveshaft is to your right so it's possible a vibration you think is coming from the right wheel is coming from your more immediate right. Ask me how I know ...
http://www.peachparts.com/shopforum/showthread.php?t=252017

-It could be a stuck caliper or warped rotor, although you likely would notice that very much on braking.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)