Re: vibrations in first gear
Probably not an adjustment problem. You did say lots of mud and sand. Those two, even with a properly geared vehicle can burn
up a clutch. You may have done a little damage to the clutch, on its way out now. The shuddering on moving form a stop is a sign
of uneven initial engagement.
Brad (from the 4 Wheeling center of the universe, 4 corners USA)