will not shift into 4wd hi

I have a sonoma pk-up same as s-10. Will not shift into 4wd hi,no lights flashing nothing but when I put it into nuetral 4wd low and hi flash 3 times then it goes back to 2wd. It will shift into 4wd low when in nuetral with no real problem.Please give me some ideas of what it could be,have no time for this now with all the snow and moving.I have the 3 button 4wd switch 2hi 4hi and 4lo so I asume its the 233 transfer case.

When i have been reading alot of other post will try a few things this morning and let ya now

Does the actuator under the battery have manifold vacuum, (at least momentarilly) when you select 4WD with the engine at idle?

When vacuum is applied to the actuator, does the actuator hold vacuum?

When vacuum is applied to the actuator, does the actuator pull the cable?

Yaeh the actuator was working I believe since it shifted into lo and you could here it. Anyway I did the reboot method I read about,(diconnect the battery for a half hour and touch terminals and everything seemed to work fine in the driveway)will let ya know if it happens again.A previous post described to a "T" what my sinario was.left in 4hi overnight was hard to get back to 2wd and then 4hi would not work only lo would engage.Although some of the vacumn lines look a little questionable I will address that in the spring.I hope it makes it till then.Thanks for your Help old master

The actuator under the battery is silent when it operates as is the clutch mechanism in the right front axle. You may have heard the encoder motor shifting the transfer case but if the actuator doesn't engage the clutch on the axle shaft, the TCCM will command the encoder to shift back to 2WD. The best test for the actuator is pull the battery out so you can get at the actuator and manually apply vacuum to it so you can visually check the actuator operation. BTW what year is the vehicle? There may be a problem with the vacuum switch not supplying sufficient vacuum to the actuator, that could explain why it shifts when you're in neutral.