Guys - new to forum - have an '01 Jimmy with 36K mi and rarely use 4wd. Have same ol' problem - 2WD fine, 4LO available (only while in Neutral), no 4HI at all. I do notice that backlight on the 3-button switch is dimmer for 4HI than (2HI and 4LO). Have pulled all the fuses(4WD, ATC), done the 5 key-switch on/off, lots of blinking, no joy. Assume I can't trace all the vac hoses, and instead, decide to take it to GMC Service shop. What are they going to sell me is the problem and the fix? Cause I know it's no $15 part replacement. You take it to them and it's $800-$1K before you leave the place. Can you give me any defenses to fight the Service Mgr. shpeal? Would really appreciate it.