4WD engagement problems

I've read the posts about 4WD not engaging and the actuator being the culprit on older GM vehicles. I have a 1994 GMC 2500 Diesel 4WD which has worked fine in 4WD until recently. It will engage but when I start driving, it pops out, re-engages, and pops out again. What do you think? Still sound like the actuator problem?