The fact that you have normal brake pedal effort with the light on and high pedal effort with the light off suggests the problem might be with the ABS control system. I'll bet something got installed wrong.
Found this stuff in the Chassis Diagnostic Manual, maybe it'll help you figure it out...
The vehicle instrument cluster has a red BRAKE warning indicator to alert the driver to the following conditions:
* parking brake applied
* hydraulic failure in either front or rear channel
* ABS system malfunction
The red brake warning indicator is controlled by the vehicle's ‘‘smart'' cluster. There is a parallel path to chassis ground that will illuminate the red brake warning indicator:
* through the parking brake switch contacts
* through the brake warning indicator switch
* through the CAB via CCD Bus
The CAB can control the operation of both the yellow ABS warning indicator and the red brake warning indicator
* If the CAB is disconnected, the yellow indicator will be illuminated.
* If the axle type or tire size are not programmed properly, the yellow indicator will blink.
* If a system DTC is set, both the yellow and red indicators will be illuminated.
and...
3. 2. 5 WHEEL SPEED SENSORS
The EBC 325 ABS system uses one wheel speed sensor on each front wheel, and one speed sensor mounted in the rear axle for the rear wheels.
The EBC 2 uses only one speed sensor mounted in the rear axle for the rear wheels.
The sensor measures the wheel speed by monitoring a rotating tone wheel. The signal generated by the sensor and tone wheel is transmitted to the CAB.
Each sensor has:
* a magnetic/coil pick-up (speed sensor) that is mounted to a fixed component
* an air gap between the tone wheel and the speed sensor assembly
As the teeth of the tone wheel move through the magnetic field of the sensor, an AC voltage is generated. This signal frequency increases or decreases proportionally to the speed of the wheel.
The CAB monitors this signal to check for a sudden change in single or multiple wheel decelerations. If the deceleration of one or more wheels is not within a predetermined amount, the antilock module takes control. Diagnostically, the coil of wheel speed sensors have different amounts of resistance based upon the sensor type. When measured across the connector two terminals, the resistance should be:
4x4 front sensor 893-1208 ohms
4x2 front sensors 1800-2200 ohms
All rear sensors 1600-2300 ohms.
NOTE: FOR ALL RESISTANCE RANGES ADD 30% TO THE VALUE FOR EXTREME HEAT, SUBTRACT 30% FOR EXTREME COLD.
The rear wheel speed sensor, used on all vehicles have the same resistance value, but there are two different sensors. Therefore, it is important to replace the rear wheel speed sensor with the proper part.
On a EBC 325 system each front wheel speed is monitored through the speed sensor mounted at the wheel end of the hub. On a EBC 325 and EBC 2 system the rear wheel speed is monitored through the speed sensor mounted in the rear axle assembly.
The CAB will disable antilock control, illuminate the ABS warning indicator circuit via CCD, and store trouble codes if it detects a problem with any
or all of the wheel speed sensors:
* incorrect circuit resistance when checked with no vehicle movement
* incorrect sensor output during vehicle movement
* erratic sensor output during vehicle movement
Attention: TDR Forum Junkies 
well was told to look at the speed sensor in the rear end well no big deal had slack in the pinion bearing's so why not do it all at the same time looked good clean. not that I really now what I was looking for it to be bad but still no luck truck stops fine when light is on but when it's off i need about 200 more LBS on me to stop this beast! it's worse in the rain
