Did you use the rear locker much prior to doing the grey wire mod? If not, the actuator can stick. R&R the actuator and inspect. The FSM also has a very good procedure listed for testing.
Even with the GW mod, the locker will only engage at speeds under 5mph. If you get above that, it won't lock in. Also, driving in a shallow "S" pattern (gently working the steering wheel back and forth and swerving just a bit) helps to get it to lock and unlock.
Will it still lock in 4lo?