When you state you will recheck the firewall ground, that makes me think that the wires that came with the kit takes power from the wire to the vacuum operated solenoid on the firewall and runs that wire to a switch somewhere that's controlled by the new cable. Then a second wire runs from the switch back to the vacuum solenoid and grounds where the solenoid used to ground. From what I am thinking the vacuum solenoid is no longer necessary and you might have removed it completely.
You may have to back up a few feet in 2H to pop the TC out of 4X. I don't know why I think this, but somewhere in the dark recesses of my poor broken memory I seem to have a shadow of some event or another.
Let us know what works for you. I'm sure that you aren't the first or last person to experience the stuck in 4X issue.
Good Luck, L.M.
1987 YJ-4.2L-Standard Shift- Re-manufactured Carter Carb-2" body Lift-31X10.5X15 BFG KOs-188K Miles
HEI distributor with computer and all related relays and wiring removed.
No back seat.
12K Badlands winch with dual batteries.