I saw your thread last nite when I was doing more searches. Unfortunately I didn't get to do any trouble shooting during the weekend. Got caught up with my side job (7am-7pm).

I feel your pain toby..I will post as soon as I get around to it. I finally got myself a multimeter tho.. Now, just need to learn how to use this thing..lmao

Another thing I will be checking is the brake lamp switch. Apparently this also plays a part with the speed control system. Then going to check all the connections...again, this might not take within the next couple of days. Work gets in the way sometimes.
As per service manual:
BRAKE LAMP SWITCH
DESCRIPTION
The plunger type brake lamp switch is mounted on
a bracket attached to the brake pedal support. The
switch can be adjusted when necessary.
OPERATION
The brake lamp switch is used to for the brake
lamp, speed control and brake sensor circuits.

BRAKE LAMP SWITCH
DESCRIPTION
The switch is mounted on the brake pedal mounting
bracket under the instrument panel.
OPERATION
Vehicles equipped with the speed control option use
a dual function brake lamp switch. The PCM monitors
the state of the dual function brake lamp switch.
Refer to the Brake section for more information on
brake lamp switch service and adjustment procedures.
The brake switch is equipped with three sets of
contacts, one normally open and the other two normally
closed (brakes disengaged). The PCM sends a
12 volt signal to one of the normally closed contacts
in the brake switch, which is returned to the PCM as
a brake switch state signal. With the contacts closed,
the 12 volt signal is pulled to ground causing the signal
to go low. The low voltage signal, monitored by
the PCM, indicates that the brakes are not applied.
When the brakes are applied, the contacts open,
causing the PCM’s output brake signal to go high,
disengaging the speed control, cutting off PCM power
to the speed control solenoids.
The second set of normally closed contacts supplies
12 volts from the PCM any time speed control is
turned on. Through the brake switch, current is
routed to the speed control servo solenoids. The
speed control solenoids (vacuum, vent and dump) are
provided this current any time the speed control is
ON and the brakes are disengaged.
When the driver applies the brakes, the contacts
open and current is interrupted to the solenoids. The
normally open contacts are fed battery voltage. When
the brakes are applied, battery voltage is supplied to
the brake lamps.

ADJUSTMENTS
BRAKE LAMP SWITCH
(1) Press and hold brake pedal in applied position.
(2) Pull switch plunger all the way out to fully
extended position.
(3) Release brake pedal. Then pull pedal lightly
rearward. Pedal will set plunger to correct position
as pedal pushes plunger into switch body. Switch will
make ratcheting sound as it self adjusts.
CAUTION: Booster damage may occur if the pedal
pull exceeds 20 lbs..

If all else fails, looks like I have to take it to the dealership. From my read and from the factory service manual, a dealer issue scan tool is the only way to diagnose this issue