I never heard of that before on GM T series, and the factory shop manual makes no mention of it either. I've heard of people raising the front wheels off the ground while making the adjustment which might make the actual adjustment a little easier. The actual measurements are taken with a normally loaded vehicle*, 1/2 tank of fuel, on a level surface, (usually an alignment rack that has been leveled using a transit). Most alignment shops don't even check chassis ride height when doing an alignment, unless you ask. Even then, I wouldn't be so sure they know what it is, much less what the dimensions are, how to measure it, or how to actually adjust it

*If the driver and or passenger are over 180#, by rights, they should be in the vehicle when you take the measurements.

 

Thanks aa21830 - The interesting thing is that it still does this when the vehicle is warm - if I back into a parking space, it almost acts like 4WD is engaged on dry pavement, but nowhere near that bad (you know the jumping that the vehicle does if you try to engage 4WD on dry pavement?). I had it flushed when I had the new engine put in.....I don't mind flushin it again if that's what I need to do, but I need more to go on.,, especially with the Service 4WD light illuminated.

And yes, it is electronic. I have the 2-Hi 4-Hi 4-Lo switch with the round "Auto" button.

Thanks,

Jonboy

 

The jumping you're referring to when in 4WD is actually called "crow hop" Might want to check the vacuum line at the actuator under the battery tray. In 2WD there should not be vacuum, in 4WD there should be manifold vacuum present. Very common for the transfer case vacuum switch to stick in the 4WD position, or leak vacuum past the seal inside the switch allowing vacuum to be applied to the actuator. The switch is located on the driver side of the transfer case, inboard and above the front output shaft:



Dorman OE Solutions 600-500 - Vacuum Control Switch | O'Reilly Auto Parts

They also market the exact same switch in the HELP! line: Dorman Help! 49315 - Transfer Case Switch | O'Reilly Auto Parts

There should not be any fluid present in any of the vacuum lines. If there is, it must be cleaned out for the 4WD to operate correctly.

On your 1999, there is an additional electro-vacuum switch mounted to the firewall:



Dorman also makes this switch: Dorman OE Solutions 600-104 - Actuator Valve | O'Reilly Auto Parts This one's a little pricey so check it before throwing a new one on.

The clue here was your Service 4WD light illuminated in 2WD. Any one, or more, of these things can cause the front axle to be engaged while in 2WD, which is a problem, the TCCM recognized it, and turned the light on.