My 99 has the 9.75 with a factory ls rear axle. it works in all gears. 2w, 4wh, 4wl. Though, mine also has the A4WD in the 2wd spot.. It runs in two wheel drive in till the rear tires spins to much. Then the computer detects it and will engage the front drive axle for the added power. If it keeps the front axle engaged for a small spell. It will lock the t case in 4wd high. Then I will have to click the switch to 4wd high and back out. If I need low then I have to switch it to low.
Hear is were I got some of my info for my 99 expy. Read the tag and look it up. Dealer helps a lot as well
The rear ls or lsd (which does the same in theory ) will be their un till their is like 200 lbs of torque or so. then will slip.. THESE ARE NOT FULL LOCKING SYSTEMS. WHEN ON GOOD ANGLE THEY WILL SLIP. HENCE THE LIMITED
SLIP DIFF. If you want full wheel lock. A locker like ARB (powered by air) lock right, Detroit, spools, mini spools. these will act TOTALLY different. They lock both rear tires together with very little give. They do ware tires, axle shafts out. I have an old land cruiser that I use off road with lockers in it. I would not recommend a full locking unit for mainly road driving, like pulling trailers or 99.99% on paved roads. But an ARB Air locker works great in these and many apps. With an ARB unit. u will have an on board air compressor for airing up tires if the go flat or u let out the air for off road driving. It really gives u a good both worlds unit.