Yup. If the ABS is activating, it's because the speed sensors are unhappy; one or more of the tires are skidding so the ABS kicks in to get 'em rolling again.
RWAL is a very compromised ABS "solution". Realistically, it's a "variable proportioning valve", best used to compensate for loaded/unloaded brake performance on the rear wheels. When it works as designed--and that seems to be fairly rare due to A) air trapped in the system, B) parts/electronics failure, and/or C) shiitty 254mm rear brakes enormously out of adjustment--it's just as I described. Totally transparent until you need it; and then noisy/vibratey as it works to keep the truck controllable--back-end not sliding around. There's no fluid pump, so the brake pedal sinks as the RWAL cycles. Not optimum by a long shot.
There's a HEAP of RWAL-equipped trucks that stop better when crammed into 4WD than they stop in 2WD. Including my '88, for years and years after I got it in '97. (RWAL is disabled in 4WD.) But better braking when RWAL is disabled not "correct" operation. It's common, but not correct. There's something wrong with it, that hasn't been properly fixed.
The 3-channel systems are not so different from the ones you listed. In fact, that Envoy 4-channel unit is merely a development of the second-design 3-channel unit used on the GMT400; the difference being that the two rear wheels are controlled independently instead of together. But the "guts" of the system are related.
ABS in the earliest years was poorly marketed. They made it seem like ABS was intended to make "ordinary" drivers into threshold-braking expert race-car drivers. ABS wasn't able to threshold brake like a skilled driver for decades after it was first installed on passenger cars. Some (Me!) might argue it still can't threshold-brake like a top-flight driver. ABS is primarily intended to allow steering/directional control during panic braking by "average" drivers. Any improvement in stopping distance is welcome, but a secondary benefit, and most-applicable in practice in comparison to locked-wheel "idiot" drivers.
Having said all that...ABS as installed in the '90s GMT400s wasn't that bad. The main issue is that it didn't work right due to what were "fixable" problems; but they weren't "fixed" properly, or at all in many (most?) cases. The trucks developed a terrible reputation for "weak" brakes. It is--and was--a mistake to require scan tools to properly bleed ABS units. The things should have bleeder screws that ANY IDIOT can manipulate. Brakes need to be serviceable, and not just by "experts". But nobody asked me for permission to require scan tools for bleeding.
At this point, there's no source of new or rebuilt ABS units for GMT400s; all replacements are via Treasure-Yard "pick-and-hope". Pick one, remove it from the donor vehicle, and hope it works right when you install it in your truck.
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