Will not shift from 4wheel drive

Just to confirm, you don't have an electrical 4WD selector; you have to move a shifter on the floor of our cab? If that's the case, I'd suspect that you have a linkage misalignment or malfunction that doesn't allow the 4WD selector to completely return to the 2WD position. Putting the truck into reverse and back to drive probably causes a jolt to the 4WD linkage which disengages the transfer case from the driveline. Check for slack or binding in the transfer case shift level linkage (clean and lube the linkage won't hurt).

the vacuum lines run to the hubs behind the brake rotors on the front wheels

service the car may help,,

The service 4 wheel drive light is indicating a part failure in the 4 wheel drive system.
Take your vehicle to your local Auto Zone or Advance Auto Parts and ask them to scan your Tahoe for trouble codes. The code will help to determine what the component failure is and you can start diagnosing at that point.
Write the code down and reply back, we can go from there.
Good luck.
Regards,

Buy a chilton`s repair manual there better then haynes repair manuals

4 wheel low engages only when u shift trans into neutral or park never when moving it can cause transfer case damage.

push button 4-wheel drive??? If so maybe the transfer case motor? look on back of transfer case for something that looks like a window motor.... have a friend, while on level surface in nuetral with brake depressed, try pushing 2-wheel button while you lightly tap on side of window motor with light weight hammer or wrench. If this doesnt work then motor may be bad.

HAVE YOUR PLUNGER FOR THE DIFF CHECKED IT SHOULD BE UNDER THE BATT TRAY

the shifter accuator or the wires going to it on the transfercase..gl

its a possibility that the transfer case might be going out or the fluids in it are bad sometimes after alot of mileage the tranfer case will slip or stick