Auto hubs not disengaging...

Have been running the truck quite a bit in 4hi this last week after not using 4x4 too much this summer.

Hubs do not seem to be disengaging...I have heard this is a common problem with the auto lockers, but what is the cause?

Any idea where to start?

Try puting it in 2wd, driving forward about 20 feet, then backing up 20 feet while turning slightly side to side. If there's tension on them, they won't unlock. The backing and turning takes the tension off. You will probably want to rotate your tires if this fixes it, even 1/32" will cause it if there's enough traction for the tires not to slip.