how do you disconnect the 4 wheel drive on a 1999 GMC sierra, mine is stuck in 4 low i think i just want to disconnect the whole 4 wheel drive

Not sure how close my 03 Sierra 1500 is but, behind your transmission is your transfer case. Coming out of the transfercase on the driver side is the front drive shaft. (Its a quarter of the size of the rear drive shaft) it runs from the transfer case to the front differential. Mark with chalk or a crayon the orrientaion of the drive shaft to the differential flange. Remove the boot clip and slip the dust boot off the transfercase. Remove the four bolts from the universal joint and pry gently with a flat screw driver. You could lightly tap it with a hammer to loosen rust. Slide it forward supporting both ends and your done.

if your stuck in 4wd low< disconecting the front shaft wont do anything, being in 4 low limits ur speed going forward your never supposed to exceed 25mph. to get it out of 4x4 theres a shift acctuater on the case (depending on year of truck and options) youve got to get a power souce to the motor to shift it out of the 4wd low (not sure on the exact schematic) if still no luck you"ll have to have a garage fix the issue of being stuck in low range, i have see the contacts on the older truck 88-94 get dirt and loose connection with the motor and simply need to be cleaned and/or replaced

I had same problem. I had to slow vehicle to 3-5 mph switch to neutral hold button till light flashes on 2 wheel drive.Put truck into park after rolling a quarter of a mile or more.Push and hold 2 wheel drive and put into drive then only go 3-5 mph. keep repeating the process and it will eventually go back in 2 wheel drive. Good luck and be minded of low speed.

Cleaning all electrical connections. Regrease and try again. Same problem happens in 04 Durango.