I am always chasing vibrations like that. Shakes, shimmies, pulsing vibrations always seem to start above 45mph. Some suggestions made to me by friends and oher car owners: flat spot on a tire or outta balance, bent wheel, drive shaft outta balance, broken leaf spring, wheel bearing(doesnt matter if you had replaced in the last few thousand miles either, any number of parts in the rear differential...then you might move forward to the tranny for possible sources. I never seem to be able to get everything smoothed out for more than a few thousand miles when something else starts making noise. Then you got yer front tires of course....try wheel bearings first. Its cheapest place to start! Unless the axles have been affected, which was suggested to me as well. And of course check wheel balance.