Whether or not the service light is linked to the functionality light of the 4wd is questionable. It should not be. If it is, there is an electrical problem in the system as @csa indicates. So… When in 4 wd on a lift, you can rotate the wheels then the transfer case shaft to see what is truly engaged. If every thing works, it’s not an actuator but a position switch problem. This assume oll connections have been checked.
You never indicated that the vehicle actually failed to function as a 4wd when the lights did not go on. If it did, it again, is a position switch problem. If it did not, the actuators ( there may be two, transfer case and front differential) could be a problem. This often occurs from lack of service And/or lack of use. Get it serviced and use it frequently as the manual suggests.
. Bottom line…you should be using 4 wd often when you don’t need it so it will function when you do.
These switches and actuators are very suspetable to failure do to lack of use and service. The old manual ones with manual hubs just got stiffer to use. The automatic actuators just don’t have enough power when you neglect them… Sometimes, service (fluid change and use) is all that is needed. Sometimes putting in a warm garage can get it to work too…but it could be just temporary.