Stuck in 4lo

I started the car the other day and had a beeping sound and a flashing 4lo light. I drove a couple minutes and it didn't stop. I stopped the care and moved the switch from 4Hi to 4Lo. The truck then took a minute but set into actual 4Lo. The difference in driving was obvious. Once in 4Lo, I drove a bit then stopped, set the car in neutral and tried to position the switch to 4Hi again, then put it into gear and drove forward. It won't come out of 4Lo no matter what I try. I'm hoping to find a way to test the actuator wiring harnesses when the switch is thrown to see if I see power using a multi-meter, or trying some way that you can suggest for me to work through the system to see if the switch is sending a signal to the actuator. It's strange that it worked once to get into 4Lo before not being able to switch back to 4Hi. Thank you!