Outer Limits 4x4 Board

I have a 91 gq 4.2 petrol. Runs like a dream;idles on 600 rpm;no misses.
problem/
As soon as you engage H4 or L4 engine will not run or idle unless you constantly keep stabbing accelerator pedal. If you dont it stops.As soon as you dis-engage H4 or L4 it runs like a dream again.Have tested this on last 3 outings ;last 1 being this afternoon;
IT is shiting me.

Can we have some more vehicle info please??

Auto/manual???
Any other mods???

I assume that H2 is ok then, does it restart in H4 or L4.
If not then it sounds like it could be a wireing short from the transfer lever dropping out the ingition and causing missfireing. You could try it with the wire loom unpluged from the transfer lever switch on the side of the box.
Other than that NFI

It is manual. H2 is fine.
Was leaning toward electrical short in 4wd wiring to switch on dash.

that would be my first place to look

go to the auto elec lube up and bend over