Obs Tahoe issues

My best friend just got her first 4x4 vehicle, a 1999 Metallic blue Tahoe LT with the 5.7L. It has 214k miles. (I believe this to be my grandpas old tahoe as it has a scratch I put in the rear door years ago with bike handle) and also has the mystery switch on the dash grandpa's had and we never figured out what it was for. anyways, it is clean and drives rather nicely, everything else works the way it should, how ever, the brakes are toutchy/grabby as hell. all you have to do is toutch them and it locks up. ABS does not come on, ABS light or brake lights on the dash are not on, ABS fuse is ok, brake fluid is where it should be, pads look good, and the rotors look fine. wht else could it be? Brake booster? I am doing the work on it, as she really cant afford $70-$80 an hour shop rate around here. I have fixed many brake issues bafore, blown lines, air bubles in system, calipers, rotors, pads etc, but havent come across anything like this. they do not pulsate, no grinding, no squeeling, they just lock up. the only way to stop it some what smoothly is using the parking brake. any help is appreciated, if anybody has had this issue or knows what it might be.
 
Idk if these vehicles have them but we had a problem on my race truck with the pressure switch for the brake light. Get on the brake at all and the rear tires would lock up tighter than a bitch. Idk if these have those or not but figured I'd throw it out there.
 
Proportioning valve maybe? My brothers did that with the insides of the lines collapsing. Brakes are normally shitty/spongy haha
 
I forget the excact part names but its basicaly the abs control unit modulator valve that mounts below it. Its not uncommon for them to fail on obs trucks. Way to be sure is pull abs fuse and see how it drives without. Last I checked it was around 600 dollars for the control unit and valve
 
None I'd my vehicles have abs. Two came with it and I just ran a switch to the fuze so if I got in an accident they wouldn't see the abs fuze gone.