This question is going to become more and more common with the cold/foul weather coming after a warm summer of 4wd inactivity.
During the warm season, there are detection switches in the transfer case and front axle that may stick due to lack of use. They need to "jive" with each other for the ABS ecu to know what mode to set itself into. If they don't jive for any reason, the indicator light will flash in the dash and if the right switch is tripped, it may also cause the ABS light to come on.
Just because they're stuck doesn't exactly mean they need to be replaced. Try shifting in and out of 4wd to free them up. Do this AFTER a long drive where the gear oil has been warmed. Allow a few blocks of driving in each mode especially if it doesn't switch over right away. Some may free up right away, others may take longer- BE PATIENT! Anticipate having to do this in the fall and winter months or prevent this with occasional switching over the summer. If your ABS light is on, continue to cycle through both modes until you feel you've made an improvment. To check, you'll need to shut off the engine in 2wd, start it back up so that the ABS light is off, then shift back into 4WD to see if the ABS lights comes on again.


Joel, CA