Service light on

That could be many things. Which engine do you have? Is it 2wd or 4wd?
Any strange noises or other problems like not shifting correctly or loss of power?
The common cause is a failure in the solenoid pack but the only way to be sure is to get it scanned by a shop or dealer so they can rear the codes and try to determine what the cause is. There are also a couple fuses that can cause problems but need the above information to be sure we are looking at the correct electronic diagram.