If the BRAKE and ANTILOCK lights are both on, there are codes stored. What are they?
Jumper ALDL A (top right) to H (bottom row, second from left). Turn on key. Watch ANTILOCK light. It flashes codes the way the ECM flashes them. Code 14 is "normal" for AWD BPMV's. Just like the ECM, each code flashes 3 times, then the next one. The sequence may not begin at 14. I watch them go by until I see 14, then begin writing until I see 14 again. This way I know I got them all.
For a Syclone, you need to find an ABS unit from a 1991 or 1992 4-door Utility (Blazer Jimmy Bravada) or 1992 2-door Blazer / Jimmy. Only these early years have the 4-sensor electronics compatible with the Syclone and 1992 Typhoon. AND: A 2WD or 4X4 BPMV from these years has to have a jumper wire placed in the 10-pin cavity D from the pnk/wht wire in Cavity A. This places the BPMV in "4WD" mode which is the same as AWD. (Close enough, anyway.) This unit was used in Astro / Safari vans too, but the calibration is inappropriate for SyTy.
It is possible to transfer the forward electronics module (the front 2" of it with the electrical connectors on it) which contains the AWD calibration from your unit to a good working hydraulic portion from any year / model donor vehicle that used the K-H 4WAL system. I have two BPMVs that could be used for this. This tactic will also allow using 93-up and Astro/Safari hydraulic units which expands the population of suitable donor units.
Begin with pulling the codes. There may be a LOT of them. Post them here, and I'll take a look. Only a few codes are "fatal" and require BPMV replacement. Most codes are set by external problems like wheel sensors, grounding of the pipes to the frame, and specifically bad or corroded connections in front of the battery where the big power feed and ground wires for the BPMV connect to the Battery cables.
I've been on SyTy net since 1998. People that understand the ABS have been far and few between. I suspect "techs" in repair shops aren't any different, especially with 25 year old systems. It doesn't surprise me that your guy can't figure it out. Few people over the years have been able to.
Pull the codes. Post back. I'll try to help.