Is my 904 toast?

Update:
I took the car out to put her through her paces on a long and straight road. Problem is worse.
I warmed up the engine on the trailer.

Went out in first and stayed in first through about 5000 rpm, then coasted back down again.
Then repeated in second, stayed in second.
Then third, staying in third.

Then came back, going through the gears.

At the beginning, in first gear, everything felt pretty good, although I think I started to feel that pulsing again as we got up to about 40-45.
Second gear, the pulsing vibration came back and seemed to get stronger and more constant - that is to say the "pulse" got longer and longer until it was a steady vibration. Low pitch, could feel it in the whole car.
Third gear, vibration turned to a roar and I could hear other things vibrating - I think even the exhaust was banging against something (it's pretty tight to the body on the left side).

The faster we went, the worse it got.

It sounds like it is coming from behind me (somewhere).

I did not feel the vibration more in the steering wheel than anywhere else.

It occurs to me that as I went further, everything was probably heating up more and more.

The engine and transmission worked together as normal and pulled like normal. Nothing felt like it was slipping and definitely no wheel hop.

I came home and pulled the driveshaft. U-joints are in great condition (and everything was nice and snug before I pulled it). I could see no evidence of balance weights missing - this drive shaft got new u-joints and was balanced right before I started racing it in November. I put a straight edge on it all around and it is straight.

The flex plate looks okay to me. I don't see anything missing or anything to suggest that it has come apart.

At this point I'm wondering if it might be in the rear end itself. The 8.25" sure grip rear end got new axles and a new 3.92 gear set last summer, done by a professional I have no reason to distrust. It has always had a little bit of a whine, though.

One thought, although it is a bit of a pain in the neck: I have the old 8.25" sure grip with 3.55 gears on my Barracuda and it is in good shape. I suppose I could swap out the rear ends and that would answer any questions about the rear end. Actually, for that matter, I have the Barracuda's original 7.25" sure grip small bolt pattern 3.23 rear end in my garage and possibly the driveshaft that went with it.