Probably a tire weight has fell off if problem has just started or belt broke in a tire. Locate which tire it is if you can. If you feel it in the stearing wheel it will most likly be on front. Wherever it is , say on front change it for one in back and see if the problem shifts away from the stearing wheel. If on back place on front , then you can feel it good in stearing. Be my first thought otherwise if a humming loud could be a bumblebee trying his best to run with you and 50 to 60 mph is as fast as he can fly. If that be the case he is asking you to slow down. :bs:
If the vibration can be felt in the shifter, it is likely a front of the driveshaft balance problem or a bad u-joint. If you feel it in the back seat or in the floor at the rear, it could be a rear of the driveshaft balance problem or u-joint. However, a bad rear tire or drum can cause the same vibration in the rear. Before pulling the suspected driveshaft, swap the front wheels for the back and back to front or borrow a known balanced set of wheels to test drive the car. If you haven’t balanced your wheels for a while, it wouldn’t hurt to do so to ensure they are indeed balanced, just for peace of mind.
Before pulling a suspected out-of-balance driveshaft, check to make sure the u-joint is properly positioned in the yoke. Some yokes have small tabs that the u-joint caps must be indexed behind to hold the driveshaft in the center of the yoke. If you removed the u-joint caps for any reason, did one of the needle bearings fall over when you put the cap back on? This little oversight WILL cause a bad vibration.
BTW - driveshaft and wheel/tire issues do not feel the same, as the driveshaft is higher frequency by a factor of your rear axle ratio. So, if you have 3.00:1 gears, a tire shake should be perhaps 12 hertz (12 times a second) at 60 mph, but a driveshaft vibration would be 3 times faster at maybe 36 hertz. A wheel shake is like a machine gun. A driveshaft vibration is like a big bumble bee. That's why driveshaft issues are often described as a buzz or hum. HTH