It's running rich. Sounds like all plugs had black carbon, so a sensor is in need of replacement. Check the codes in the computer for something obvious.
http://www.troublecodes.net/Ford/
If the codes come up 1..1....1..1, the computer couldn't find any electrical faults. Here's where it gets tough. Wiring diagrams make it easy though. I don't know you're specific wiring because they changed colors throughout the years.
The fuel pressure regulator could be shot. Check for fuel in the vacuum line running from the vacuum splitter to the FPR on the fuel rail. (Driver's side, near firewall) If you have gas in that line, the FPR diaphram is ruptured. Not all bad FPRs go out like that, you'll need a fuel pressure gauge to really determine if you have a fuel pressure problem.
Check the voltage from the o2 sensor. **Though, the o2 normally won't cause an issue until after the truck reaches operating temperature. It won't read until the o2 is heated. (2.9L Rangers have HEGO) Located on the Exhaust y-pipe before the catalytic converter.
I believe there is a table for resistence vs. temperature on the ECT in one of the Hayes or Clilton's manuals. Check this. This sensor will also cause a rich mixture if it's bad. Located on the lower intake manifold, passenger side. It's the engine coolant temperature sensor. Not the gauge sender. The gauge sender has 1 wires, the ECT has 3?
The IAT will also cause a rich mixture if it fails and thinks it's cold outside. (cooler air = closer air molecules = needs more fuel) Located on the passenger side of the upper intake, I believe the middle runner.
Check the voltage at the TPS. If the TPS is giving a higher voltage (should be around .9 at idle, 5.5 at WOT), it will give it more fuel because it thinks it's getting more air. Located on the passenger side of the throttle body.
Lastly, check all of the vacuum lines and pay close attention to the MAP sensor. I converted my Ranger to MAF, and really generally don't have any clues on how to test the actual MAP. Usually located on the firewall, passenger side. Little black box.
There are other factors, but these were the most common that I could think of off hand.
Pete