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start by checking your vacuum system, the line coming off the vacuum pump is a common problem, check all lines, and ends. good place to start.
1999 4x4 qclb,quad adrenaline v2,auto to nv4500,SB 3250,3.55s,air dog 150 with big line kit,weston 8X11 sticks,head studs,BD 64.5/74/14,boost/egt/fuel pressure,2 inch lvl kit,35's on 17 pro comp wheels,home made trac bars,5 inch miter stacks,fresh paint
alot of times it is the switch inside the t-case. Pull the vacuum lines off the cad on the front axle. There is a red one and a black one. while in 2wd it puts vacuum on one side and in 4wd it puts vacuum on the other. (i forget what color is for 2wd and which for 4wd). Have a buddy in the cab with truck running go under with a vacuum gauge and check each while he shifts from 2wd to 4wd. The truck needs to be running, and yes it is stupid friggen loud under there.
I had the same problem also and if all your vacuum lines check out its most likely the 4wd actuator on the front axle. Not too expensive or hard to replace I bought mine from geno's garage. My actuator got stuck in the 4wd position.
The rod in my 4wd actuator was getting jammed in the case in the front axle. I disassembled it all, cleaned it and drilled the guide hole out a bit. Put it all back together and it is working great again. The seals in my actuator are bad without a doubt, but this has given me a little more light out of the 4wd. I will just do a manual setup at some point here.
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