Wierd Noise when above 50 mph

[HuskerFan]

Hey guys, wanted to make another post as i have another question now that you guys helped me identify that i have the turbo 350 tranny and 203 t-case. When i got the truck, the guy told me it was geared low, but he didn't know the ratio. He said to keep it below 50-60 mph. So when i got it home, i jacked up the rear end, put it in neutral, chalked one tire, and spun the other 10 times and counted the number of revolutions the driveshaft made. Worked out to 4.875, so 4.88 gears. Ill attach the link for that video. Anyways, when i get to about 55 mph or higher, theres a noise that sounds that sounds like it's coming from the tranny. I took a video today at 65 mph and you can hear the noise. Tac was at about 2800 rpm or so, as you can see. I'll attach a link for that video as well. Any input helps! Thanks Guys!

Gear ratio: https://www.youtube.com/watch?v=aYXs7DK9EiE

Weird Noise: https://www.youtube.com/watch?v=_a_2yhAhKZo