I'm by no means an ABS expert, but I've got some TSB notes on ABS diagnostics that I thought might help identify potential problem areas from the symptoms you've given.

The closest basic fault types to what you describe are:
Type 5 -Fault occurs sporadically while driving, stays lit until ignition is switched off
Type 6 -Fault occurs during braking, stays lit until ignition is switched off
Type 7 -Fault occurs during braking, stays lit, but goes out before ignition is switched off. This would seem to be the one that matches your symptoms.

For fault type 7 where the ABS system is still functional, then the problem is listed as poor alternator charge. The ABS dash lamp lights when the D+ wire is shorted to ground. As it happens to you when braking, I don't think this applies.

For fault type 7 where the ABS system stops functioning, the problem is listed as an intermittent/poor power supply related fault. It says this can involve: a bad battery, poor alternator output, bad grounds (ABS controller, sensors), faulty wiring (ABS controller), bad fuse in transient protection relay or bad relay itself, and finally a faulty ABS controller. From what I can make of the info and your situation I would be very interested in the transient surge protector relay (I presume that's in the dash relay tray).

If the above checks out okay then here are some general notes. Most ABS problems are at the sensors or in the sensor wiring, especially the front sensors, and not the controller. That the diagnostics pointed to the rear sensor may be a red herring. Obviously the sensors need to be clean and undamaged (check the sensor tips and back mounting). Same for the pulse wheels which should also be checked for warpage. The sensor wiring must not be damaged as the integrity of the shield protects the tiny AC current from RF/EMI interference -that also means keeping all ABS wiring away from ignition wiring and modules such as the coil. Check the sensor wiring in the wheel wells for damage, especially where it goes through the fender. The sender connectors in the wheel wells should be opened, inspected, cleaned (spray contact cleaner) and re-sealed (dielectric grease). Check and clean the ground at the ABS hydraulic unit. Check and clean the ground for the ABS controller (ground rail at left A-pillar, behind kick panel). Your rear sensor wiring has been checked, but you could check it again for damage at the critical points. I don't have the routing handy, but I presume it lives in the left rear harness so I would want to remove the rear seat and check the area where that harness comes out over the wheel well and starts to head up next to the rocker.

Other thoughts:
-Is the speedometer working properly with a steady needle? If so then the basic wiring connections to the rear sensor should be okay.
-When the new rear speed sensor was put in, was the clearance set properly? For a 48-tooth pulse wheel it should be set to .6 mm (min .3 mm, max .8 mm). That appears to be for all later (1989-on) ABS systems, but you'll need to verify that. Some sensors were mounted in aluminum differential cover plates and are not adjustable.
-Are the front tires stock size and are they properly inflated? If they're radically different from stock then the rolling radius could create problems. The ABS systems are known for being more sensitive at slower speeds and this might explain your intermittent problem. There are a number of tire calculators on the internet such as at tirerack to verify that.
-Imcomplete brake bleeding can also cause ABS problems.

Hope something in all that helps you and/or your mechanic.
--
Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day