Here's the latest scoop. Thanks for the suggestions thus far.
I think though the vibrations are similar, it seems the vibration at 75+ might just be normal wear/tear. It's not really that bad, and shows up all the time at that speed.
However, I was not hearing the noise or feeling any vibration after leaving work today. No noise at rev in Park, and nothing at 60. UNTIL....I drove it around a while. Then the noise/vibration were both back. So, the truck needs to be "warmed -up" pretty good to reproduce the problem. When I returned from driving around, I tried revving the engine, and noticed it is a quite loud, low-pitched hum. It seems to be coming from right around the firewall area, but it's really hard to tell.
Would the torque converter vibrate in park?
I am really lost considering the fact that I need to drive around awhile before the noise shows up.