C/D is for Center Differential Lock and is different from 4WD. This locks the front and rear propeller shafts into one and under such circumstances, when making turns on hard ground, the front and rear wheels would be turning at the same speed and cause the vibrations and grinding noise you hear and feel.
Something is wrong with the C/D so you need to have a look at it.
Saturday, June 16th, 2012 AT 6:48 PM