I once owned a 1986 Chevy 4x4 pickup ( don't hit me , it went with the ex in the settlement ). Anyway, I had similar problem, searched and searched for the cause. One night a friend of mine and I were out in a local parking lot -- very loose gravel. I put it in 4WD and cranked the wheel hard to the left. He said he could see some sparking from behind the wheelwell on the right side ( battery on the right side, too, btw). Turns out the battery cable was rubbing on the inner fender in turns and was shorting, intermittently, laying on the fender sometimes and not on it, others. It didn't appear to even be damaged,but upon closer inspection a fine crack in the insulation had worn thru. Just luck that we found that one. Seems like you just got a short somewhere.

Mike in AR