2008 4wd periodically not working

Had this truck for 6 years now, nothing wrong! Love it!
But the past few weeks, when switching from 2x4 to 4x4 (and back) it wont do as its told.. It often takes from a few min to a hour to switch over.
Where do I start looking?

2008 5.7 (with LPG) aut. 200k

 

My 08 does the same thing. Not sure why either, I put it in neutral and switch over and dont hear it engage?

 

Changed your dif fluids as scheduled?

Driven it 10 miles in 4x4 mode ?

 

I am putting in 4.88s tomorrow, So the fluid will be new, I will give it a try, Im wondering if its something with the transfer case not engaing properly..

 

A few minutes to an hour??? Say What??

 

Does your 4X4 light on the dash keep flashing??