I had a theory...

[Original_Ranger84]

Ok so i have the vacuum hubs on my 99. I was thinking if they run off of engine vacuum and I stall the motor to low rpms while driving (5speed manual/3.0L) on the beach per se. Could that be the cause of them disengaging? Because I try it out everywhere else and the hubs go in 100% of the time, just not the beach or when I'm trying to get unstuck... which is a pain cause thats when i need it. Also if i am stuck and let it sit slightly revved they seem to engage.

 

[4x4junkie]

They shouldn't be disengaging on their own. Vacuum is only applied momentarily to lock them in, and then again to unlock them (this is the "pulse" in PVH). Without vacuum applied, they should remain in whichever state they are in (locked or unlocked).

Sounds like it might be time for a swap to manual hubs.