Pulsing vibration at around 55-65 mph

[76/85 Truck]

I have this exact same problem. At around 55 it starts pulsing (vibrates, then pauses for just a SPLIT second, then vibrates, and this continues until i go below 50). And when I let off the gas and let it coast it gets worse, then stops. I've had the driveline spun and the tires rotated/balanced. Nothing helped. I'm thinking I need to check my rear end. Also, when I let my truck sit after driving it a few miles, it will go "CLUNK"...like it is relaxing or something. It'll do that about 3 times. It echoes throughout the bed and the exhaust vibrates. I'm thinking my rear end has some play in it, and once it is in "park" it just slowly relaxes the gears. Hope someone can help ya! I've been looking for over a year for my problem!! Good luck