Well, I've been WAY out of town for awhile. I've gotten most of the engine replacement issues worked out. Here's what I've got left. Let me start off by saying that the tires are freshly rotated and balanced, so this is likely n ot the issue. Also, exhaust is all intact, with no holes.
I have a loud low hum/vibration during acceleration starting around 60MPH..it fades, and shows up again around 75MPh. I can reproduce the noise in park by slowly revving the engine to around 2250rpm and 3250rpm.
However, if I am at the 60mph or 75mph range when the noise is happening, I can put the truck in Neutral and rev all I want, it does not affect the noise. Finally, the noise/vibration seems at it's worst during DE-celeration around that magical 60mph area, and is most noticable after you've been driving a while.
So, I've spent over 6000 getting the engine running right...please tell me my torque converter isn't going out or something.
Ideas? Where should I start looking?
This problem was not there before the engine was replaced; trans. fluid changed to Amsoil, and trans/diff/x-fer case drain filled.

Thanks again.
Shawn