In trying to find the source of a really annoying intermittent drive line shudder I have removed the rear prop shaft to check the uni joints and there is a large counterweight just in front of the transfer case joint that doesn't appear to be mentioned in the manual anywhere. I cant seem to be able to post pics. It is circular about 6 inches in diameter with grooves around the face with rubber rings in the grooves on the rear end there are holes that suggest it may be multi layered or that it may turn independent of the main shaft but it doesn't might be seized and doesn't seem to be removable. Any thoughts appreciated.Is this normal to be there? is it off a different vehicle? vehicle is a 2004 3.7l petrol auto

Pictures may help

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MUDD'S MOTORSPORT'S We do IFS lift installs ,
03 OVERLAND EDITION /Kilby-Skidrow-Mopar-4XGuard skids/ 2.5 inch TOTAL CONTROL JBA coilovers -JBA Arms/MOABS-31 FALKEN WILDPEAK AT3W / 4.10's etc, 04 Kilimanjaro Edition loaded, plus 05 KJ limited