Further to what has been replied, I have:
1) Had all four tires and wheels re-checked for balance and trueness...no problems.
2) Re-checked the brakes again and found no problems.
3) Pulled the trans, clutch, driveshaft, rear diff cover again and re-checked it all.
3) Put everything back together and performed another lengthy series of road tests...same problem!!!!
To review, the "vibration" is not in the steering wheel or seat, but is more like a throbbing type of load noise inside the car, kind of like what you would get from a loud resonating exhaust system, but it is not that. It is worse with the clutch pushed in and does not change if you shut the engine off while driving, in gear or in neutral or touch the brakes while driving under power or coast. There are no worn bushings in the rear axle suspension and the pinion angle is stock. This car is a stock Camaro, no custom wheels, suspension mods, engine mods, or anything else weird.
Any new thoughts????

Have you ever dialed in the bellhousing? If you disengage the clutch that is the only thing that I can think of that changes, the disk will be able to move and allow the input shaft to wobble. I may be wrong but it is the only thing I can come up with.

Maybe a brake binding? I have never seen vibration caused by drum brakes set up too tight but have seen calipers hang and cause weird vibrations in the front end. Kind of a stretch but thats about all I can think of.

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375hp 78 Chevy truck
77 Chevy Nova
95 Chevy Lumina 3.4L
and building a 78 Nova

Your car is on TNT right now. The year is wrong but only Stephen King could write about a car like yours

Have you check for a bent axle shaft? Has the car ever been in a wreck?

Can someone drive behind you and observe any wheel motion or other abnormal movements?

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Rick Dorion
69 RS Conv,355,M20,4.10's

Have you checked the wheel bearings? They can sometimes produce a resonating sound and not the common rough sound. Jack up the front, grab the spring with one hand, rotate the tire with the other. If you feel any vibrations in the spring the bearing is bad. Little harder to determine the rear bearings but jack it up put it in gear and listen closely for any bearing noise. Just a suggestion if you haven't already checked them.

My suggestions are:

1. Put car up on four jack stands with weight on the springs, fire up car and see if driveshaft, etc. turn true, same with rear axle,

2. Change all four wheels and tires with a friend's car, road test, might have a noisy tread design of present tires.

Everett 68/350/PG/11.90/115mph

i know you said it didn't have custom wheels, but i thought i'd add this: I had a very similar vibration that i chased for years. i recently read a post about the Cragar SS wheels that use the unilug bolt pattern. turned out to be my vibration problem. i ordered up a set of Vintiques rally wheels and it's smooth as can be right up through 100!