My 4wd goes in an out. The 4wd light goes on and off, at the same time the speedometer light goes on and off. Must be connected since they behave the same. I saw a post that said to take the front cover off and ground somehow. Any instructions on this?

Hi! A friend of mine is selling his explorer 400 to me. I thinks it's 1995 model. But there is some thing is wrong with it's 4x4/dual, it automatically switch on and off while running which cause it to engine brakes drastically. Is this an easy fix? What are the things I should check? Hope...