Clutch Gone?
 

Ok, just to verify what I'm thinking, and this maybe one of those horribly obvious kinda things, but I've gotten a differing opinion that I don't necessarily believe.

Here's the symptomolgy of the patient (78 280)

When driving in gear, all is well, as long as my foot is even slightly on the gas. Once I take my foot off and glide, while still in gear, really bad ginding noise coming from the general area of the trans. Take care out of gear, or depress clutch, majority of grinding noise goes away. Shifts fine into and out of all gears.

My guess is the throw out bearing.

I've been told it may be a pinion gear in the rear end.

What say you all?

All though possible I don't think it's your differential. Since the noise goes away when you press in the clutch, it's more likely the disc since the bearing stops turning. When your just driving at an even speed, everthing is turning together. When you let off, the engine slows down and everthing else keeps turning faster.

I agree with theramz. You can go buy a clutch kit for around $125 at autozone/kragen, and it comes with a clutch disc, pressure plate, and throwout bearing.