The OD can be used in any gear including 4wd, Hi or lo. There was a studibaker OD that some put in jeeps that was only a 2wd model. Look on the back of your tcase (drivers side), thats where the OD will be if you have one. It should say right on it "WARN ALL RANGE OVERDRIVE". This unit is limited to 300flbs of torque, so watch the power you feed it, and keep the oil level FULL, they are very sinsitive to oil levels. I burned mine up a month ago or so due to low oil.