You're measuring it wrong. One line is VREF (Brown/White), another is SIG RTN (Gray/Red). Don't pay any attention to the colors on the TPS, but instead look at the actual engine wiring harness. Between those two there should be +5.00V, or within +/-0.05V of that. None of these measurements should be made relative to chassis ground; the sensors are on their own little circuit, as the PCM only looks at the voltages coming back to it relative to those two levels only (not chassis ground, battery positive, etc.).
You should be putting your DVOM's negative lead to the SIG RTN line, and the positive lead to the TP line (Gray/White).
I don't have wires going to the TPS module that match the colors you describe. The wires going to the TPS plug on the factory harness side are orange/white, black/white, and green/white. This matches the 1987 chassis electrical diagram in my Chilton's manual. If I reference the 1994-1995 diagram I find that the wires correspond as follows:
Gray/white = green/white going to ECM pin 47
brown/white = orange/white shared with MAP sensor, EVP sensor, going to pin 28
Gray/red = black/white
So, I should be measuring voltage with my green/white wire as the (+) and my black/white as negative.
Here's the voltages I'm seeing:
Measuring the voltages (throttle closed then throttle WOT)
20 v scale (shows 12.00 with battery voltage)
Green/white (+) black/white (-) .00 closed .04 WOT
2 v scale (shows 1. with battery voltage)
Green/white (+) black/white (-) .000 closed .037 WOT
I'm not seeing anything close to the 5V everyone is citing. If I use a chassis ground for the meter (-) I get 5.02 volts at idle position, and 5.09 volts at full throttle position on the TP wire. The orange wire consistantly reads 5.10 volts regardless of throttle position.
Keep in mind that my truck does start and drive, there isn't a no start or hard start issue. Any idea what I am doing wrong? The OP said the chassis ground was ok to use.