In order for the ABS to work using the parts from the '91, you will need to use the transmission as well. On the early GM truck abs, the abs only works on the rear wheels, and the sensor is in the transmission, rather than the axle. You don't say if the donor vehicle is 2wd or 4x4, but the abs is only operable in 2wd mode if it is 4x4.
My personal opinion, I wouldn't bother with it. 90% of the time, the abs will be inactive. Also, most people are under the misconception that abs will make a vehicle stop in a shorter distance, it does not. In fact, comparing abs vs same system without abs, the non-abs will stop in a shorter distance. The abs equipped brake system is more about control, as it allows the wheel to roll rather than lock up, potentially causing a slide or skidding situation.
Markeb, I have experienced what you're talking about, especially on the 4 wheel GM abs systems. Seems that if they get a bad signal from the sensors, usually a front sensor, they rear it as a "lockup" situation, and release that brake. The newer ones are better, but they even still do it to some degree, but without the total loss of braking that you experienced. It's one of the bigger complaints you'll see when the magazines test GM vehicles, though they stop just fine.
