Spot

I would try putting a jackstand under one of the rear wheels so one of them is off the ground. Put a wheel chock under the other rear wheel. (this assuming you're talking about you '85 and that it has the stock open differential in it). You may want to put the front up on ramps too (so you can get under it), and then (with one rear wheel OFF THE GROUND), start it and put it in gear and slowly let off the clutch and see if the noise is still there. If so, slide under it and see if you can pinpoint where the noise is. It could be your flywheel needs to be resurfaced or the pressure plate tabs may be weak or bent. If you replaced your clutch recently then one of the tabs on the pressure plate may have gotten bent and that's what you're hearing. If you haven't replaced it recently, then it could be that they are just worn to the point where they are flexing or something in there.

Another possibility is your transmission mount. I'm kinda guessing here but it may be that the tranny vibrates more with the clutch engaged and this is transferred to the broken mount.

There's no way to tell you exactly what's wrong, especially if it's not caused by something on the outside of everything.

If it's due for a new clutch, then make sure (if you have a shop replace it) to have the rear main seal replaced while they're in there, and definately have the flywheel resurfaced. You should also have them replace the oil seal in the front of the tranny too while it's accessible.

My '84 did that for a while after I put a new clutch in and had the flywheel resurfaced but in a month or so the noise stopped. It was just a noise though, I never felt any vibration or anything else.