Most noticeable when truck is heavily loaded (full of bricks or full of wood).

Somewhat less noticeable when not loaded, or when put into 4L.

Somewhat told me there is a screw that can adjust it (he had 97 1500 manual), but I wouldn't know where to start, nor do I even get the mechanics of the judder.. perhaps plates not well aligned?

More gas seems to make it worse.

Also, when put in low gear seems to not roll/slow down even when in neutral.

Truck has 126K miles. Clutch seems OK beyond first gear.. no grinding, slippage, etc.