Remove hubs, remove circlip and take parts out and keep in proper order...until you have removed piston and return spring...then reinstall piston and spring in inverse order, than all the rest as it was. Spring will then keep hub engaged.
Avoid driving in 2WD on slippery surface, as drag from the permanently entrained front driving train is a toll on the rear wheels trying to push all of that and it may lead easily to loss of control...my experience....so engage 4WD at will as soon as you judge necessary...at any speed, since everything in this case is already turning and synchronised.
2nd plug?..can be spark plug, boot, coil....; in my case, #2 plug was improperly seated and exhaust contaminated the booth, leading to a electrically conducting bridge within the plug boot, so the spark bypassed the internal electrode and flowed on the outside of the plug.
IF you change spark plugs, use antiseize compound....
