1. Why does the 4.0 SOHC have a balance shaft in the 4x4 and not in the 4x2 ? 2. What are the consequences of replacing a balance shaft engine with a non-balance shaft engine? I have a '01 4.0L in a Trac and a guy wants it to put in an '03 Explorer 4x4. I told him there was a difference but he still wants it. Help me out with some explanation please.