That's a hard one Andy. I don't know enough about the OBDII PCM and how it can work with LPG. I would have thought that a reprogrammed PCM would be required as part of the conversion to LPG. I know that LPG is best used with much higher engine compressions, like 15:1, and the SOHC is in the mid range for a gasoline engine. A static compression ratio of 8:1 would be low, and over 10:1 is high for gas engines today. What I'm hinting at is that the best air/fuel ratio is different for LPG versus gasoline use, and the compression ratio is also part of the equation. That is why LPG conversions are rare, the best conversion requires higher compression, a much different rebuilt engine.
I agree with James, I don't believe that the PCM is defective in your truck. The O2 sensors are used to determine what the A/F ratio is, and the PCM does a fine job of obtaining the programmed A/F ratios for various conditions.
If you have a stock programming PCM, then I think a better program would be the first thing to obtain. The stock PCM will not be running the engine for best LPG efficiency, it will be trying to adjust the fuel to obtain the best gasoline A/F ratios.
Do some research into LPG conversions, specifically related to needed air/fuel ratios. Try to find a source for programmers who can build you an LPG program for your PCM. The Xcal2 is the current best PCM programmer device, also called a flasher, and made by SCT. You can buy them from many sources for $350ish new. The key is the person who installs the programming into it, which can be sent by email. Consult with Doug of
http://bamachips.com/ to see if he has heard of any special programming for LPG engines. Good luck,