Feels like bad motor mounts, but isn't. Driveshaft, maybe?

[M374llic4]

My driveshaft (i believe) is doing something weird. It kind of feels like bad motor mounts when starting out in first, but just replaced motor and tranny mounts with solid ones. Feel something hitting / bumping sometimes. I heard there may be some kind of bushing or something in that area?
(91 s13)

[the head]

There is an intermediate donut on the stock driveshaft that is probably your issue, or nuked U joints and its banging around inside the caps. Does it vibrate at speed?

[ZenkiKid]

There is an intermediate donut on the stock driveshaft that is probably your issue, or nuked U joints and its banging around inside the caps. Does it vibrate at speed?

thats my guess too.

Easiest and quickest way to fix it is to go 1 piece DS.

[The Saint]

If you have the cash 1 piece DS.
If you hear a "Thud" under your ass each time you pop the clutch or stomp on the gas.
just remember to zip tie a sandwich bag to the end of your tranny when you remove the DS or you will have a stinky puddle of tranny fluid to slither around in.