Wheel sensors engage the AWD when the rears spin faster than the fronts. For an over simplification. Many parts need to function to make it happen. So many Astro owners change out the 136 transfer case for a 233 then install a manual shifter to obtain 4x4. So many options you will have to search and read. I kept my AWD 136 transfer case (for now) I installed a cast iron front differential with a LSD locker. I have several 233, but I’m. Not in a hurry to go 4x4. If the sensors and computer work the AWD is great especially in snow. Someone who knows more will be along shortly.