Blowing fuses in 4 low

I've searched and cannot seem to find anything. The past 2 times Ive put it in 4 low I have blown fuse #21 (20 amp) in the PDC. When this happened I was going uphill with moderate throttle applied. I can beat on it in 2 and 4 high no problem, even in 4 low on flat ground it doesn't occur, but once I get on it a little bit the fuse blows. I did a pretty thorough check on the harness underneath and in the engine bay and everything looked good. It killing me as I want to go wheeling but sucks when the XJ is reliably unreliable. Electrical is definitely my weak point. Any ideas?

Oh yeah:
99, aw4, 231, 4x4, lift, 4.56 gears, locked f&r.

Must be a wire that swings against metal and grounds out when you are on the throttle.

which one is fuse 21?

ecu or fuel injection related?
check the wiring that's by the manifold.

Check the wiring behind the valve cover as well.

Fuse 21 is for the PCM/ignition coil. I know for sure when it blows the result is no spark. I have searched extensively for loose swinging wires as that was my first thought. I did find one old thread that someone was blowing the same fuse in their 97 when applying full throttle, even on the street in 2wd. They claimed a new alternator fixed it.

I'd suspect o2 sensor wiring rubbing somewhere.

I guess I could see a bad alternator causing that at full throttle, the PCM controls the field wire, but I don't know why it would only happen in 4 lo.

I've gone over the harness over and over and haven't found anything. I've let it run and moved the harness all over to see if I might make it blow again. I've even pulled the harness down that runs to the rear o2 sensor and above the trans and it looked fine. As mentioned I don't know why it's only in 4 low, but in trying to wrap my head around this alternator idea. I was thinking, this last time it blew I was on fairly flat ground trying to get my passenger front tire up and over a tree stump. As soon as I put some throttle into it, it blew but I was relatively still as I was trying to climb the face of the stump.