Diagnose this driveline vibration...
 

93 C4, Okay, at 2500 rpm and at 4500 rpm I have a vibration under medium to heavy throttle. Does not matter if AC is on or not.
In neutral, or park, the problem is not there.
Had main drive shaft checked. Good, and balanced.
Half Shafts have zero movement, and as driveshaft, has spicer u joints. They are a few years old, but very few miles.
I replaced end links on rear sway bar. Rubber was gone.
Hal Shocks, no leaks.
DRM Long tube headers since 2001 era, and OEM cat back exhaust.
700R4/4L60 Pro Built Automatic with Vigilante 2800 stall
New tires.
Center beam has ZF Docs plates, and they are torque per FSM.
When you get on it, half or full it feels like something is vibrating/hitting the floor boards. Feels like exhaust is hitting. But, when I was under there, I cant see where the exhaust could be hitting. Maybe under torque, it is. Motor mounts are tight, I just did that a couple days ago.
So, thats all I can think of.
Thoughts, suggestions?

SOLVED. I have found the issue at hand. Left side/drivers OEM exhaust is/was touching in two spots on body. The shield was touching the body.
Also, the header triangle plate was touching ever so slightly.
Its not fixed, as I need to figure out what was wrong with the exhaust to touch. Is it bent ever so slightly upward?
The header collector clamp needs trimming..