Ok let's backup a little bit.

You do not have a 4wd lever.....period.
That lever is a manual way of shifting into low range.

SHe has what is called drive flanges on the front axle,they mechanically lock the hubs/C.V. joints together.

When she pushes the 4wd button,it activates two solinoids.
One moves the Transfer case into 4wd,then the front axle disconnect moves and locks the driverside axle together.

If she drives on pavement(even wet or a real wet slush)the kind of slush that let's her tires get to the asphalt then it will bind the TransferCase up,when it binds it will "pop" out of gear and go into the condition she discribes.

This is not an AllWheelDrive system.There is no differential in the TransferCase.So if the tires are on pavement even though it's wet there will be Binding happening especially during or after a turn.The front of the truck turns in a different arc/radius than the rear.This means the front and rear actually travel different lenghts which is what causes the binding.

I hope this isn't to technical,but It can be explain'd differently.

Let me know if you have further questions.

Also this "binding" is a bad thing cause if it doesn't pop out of gear than something can break.

Bansil