what causes vibration at 48 mph to 54 mph then smooth driving

IF this happens only *under acceleration* then subsides at cruising - - [as stated] it is possible a worn axle shaft inner cv-joint is causing the problem - - however this typically produces more of a bouncing or wobbling sensation in the front of vehicle - may also be accompanied by ‘torque steer’ (shake) felt from steering wheel - *while accelerating*.. ▪️ An ‘experienced’ mechanic should be able to determine during test drive ▪️