Thank you all for your input on my question. I took the truck to my mechanic and he gave it a good go over and could not find anything wrong. He was said it might be the heat shield on the exhaust system vibrating at that RPM however he could not find anything loose. I did get the transmission fluid changed and everything looked good there. Thanks to you and the readers of Piston Slap for all the good advice.
Loose heat shields make tinny, obnoxious rattles. They can’t cause what you’re feeling. I am a little concerned your mechanic isn’t looking at your specific problem.
The big concerns raised by the Best and Brightest (torque converter, engine mounts and driveshaft U-joints) are hard to diagnose with the truck on a lift. If the mechanic didn’t drive it on the highway, gun the motor while looking at it, etc. you should take it back. Or find another mechanic. Because time is money, and you probably wasted a fair bit of both right now.
That’s my $0.02. Best and Brightest, am I wrong?
(Send your queries to firstname.lastname@example.org)