What would cause a slight vibration under power at about 50 mph or higher

it could be center bearing, if its going bad it can also cause a vibration, make sure you have sufficient fluid in the diff as well, you can also get a noise and vibration from collapsed motor or trans mounts

A full rotation and balance couldn't hurt. Possible brake issue too...

I'm thinking tires. Have the balance
checked.

I don't think any body can help you here.
any mechanic have to drive the car to experiece the problem . Impossible to fix it over the phone.
Take it to mechanic shop to prevent futher damage.

sound like the u-joints in the driveshaft. Lay under the truck and where the u-joints are try moving the driveshaft up and down. If it moves then the u-joints are bed. This will cause a tremendous vibration under a load and the faster it spins it will smooth out.

I just had the same issue with a 1999 2500HD and the vibration got worse above 50 mph. Problem was resolved with a new driveshaft. Old one (aluminum) was badly out of true and the problem didn't really occur until I had the U joints changed - then bent driveshaft vibration was noticeable. New driveshaft resolved issue.

the first thing i would check is to see if 1 or more of the tires has a bubble.

Your noise sounds like it is power train related,If you have eliminated the engine,Since it doesn't make the noise while moving and in neutral.
It sounds like a universal joint in the drive shaft.Do you notice a vibration
along with the noise?

Welcome to fixya the best thing to do is check the rims for imperfections and slight bents that causes the vibrations at that speed go to www.collisionrepairadvice.com to get a wheel repair company to look at.

A few different things could be causing this, most are internal engine components. First off, take your tires and have them balanced to make sure you didn't accidently fling off a tire weight when you hit a pothole. Next, if that is not it, either you have a blown head gasket, cracked rings, or bad rod/main bearings.