Your cell phone. Sorry, my phone just did the same thing! Possible tire tread worn uneven, do you feel it in the steering wheel or the whole vehicle, ie the seat ect? Only at say 55 mph? More pronounced turning the steering wheel one way or the other?
Not an answer but need one... I see this is an older thread but, my '03 Sport Trac started to vibrate at 50 mph and smooths out at 60. The vehicle had 120k miles on it when it started this (late-summer 2015). It had new tires with about 1k miles on them. Cannot tell if it's front end or rear. Vibrates during acceleration and deceleration. Had tires re-balanced and a new drive shaft put in with new U-joints. Still vibrates. Had most front suspension parts replaced with Moog bushings and 4 new shocks in 2010 when I bought the vehicle. Have had to my mechanic 3 times but still vibrates right at 50 mph. While I see this problem discussed on many thread but little to no actual answers.