1995 Chevy Tahoe WON'T GO INTO 4 WHEEL LO OR

Transmission problem
1995 Chevy Tahoe Four Wheel Drive Automatic

PLEASE DON'T THINK I'M YELLING I JUST LIKE CAPS

THIS IS A FINE RIG EXCEPT.
IT'S BEEN INTO THE DEALER I THINK AT LEAST 3 TIMES
YOU PUSH THE BUTTON ON THE DASH TO GO TO 4 LO OR 4 HI BUT IT DOESN'T WORK
WHEN IT'S IN 2 WHEEL DRIVE THE 4 WHEEL LO AND OR HI BUTTON BLINKS ON AND OFF

SHE'S BEEN TOLD BY THE DEALER(I THINK)
THAT THE TROUBLE IS IN THE TRANSMISSION ITSELF AND MUST BE "CRACKED OPEN" IN ORDER TO FIX IT SHE DISAGREES

SHE FEELS(HOPES) IT IS A SIMPLE MATTER I.E.
A VACUUM HOSE OR SOME SUCH
THANKS MUCH BUBBA