1996 tahoe 4x4 problems

1996 tahoe will not go into 4x4 high range. 2 different mechanics have spent 5 hrs and 12 hrs and cannot find problem. they have tried new switches, transfer case without success.
thanks for any ideas on this. bob

connect a piece of wire to ground then connect the other end to the diagnostic link at terminal 13. The terminals 1-8 are on the top row left to right, 9-16 on the bottom row left to right. terminals 4 and 5 are grounds. when terminal 13 is grounded(make sure the wire is not touching terminals you don't want it to) turn the key on and count the flashes from the 4wd indicators, it may flash 1-4 times then pause for 3 seconds and display a new code or repeat the same one