.....or possibly a transmission mount has broken or turned to mush. Then maybe a loose spring hanger or clamp. Of course, it could be a loose wheel or sloppy axle splines.

What's your idle speed? If it's above 5-600 rpm, then the clunk is just all the loose parts going from a slack position to a loaded condition.

AND you do need about 15"Hg at idle to operate right