Is it possible that the T100 ECU can be reflashed for 'high altitude' calibration? Ask a knowledgeable dealer service rep - was your truck originally sold in CO or from out of state?

Are you using OEM brand (factory P/N) replacement O2 sensor? If not I highly suggest you try that first.

The O2 feedback sensor signal is the heart and soul of modern fuel injection control systems. These sensors can be 'biased' slightly rich or lean to meet the needs of specific OEM engine calibrations. Outside the Toyota powertrain enginnering center you will never know what the techies had in mind when they spec'd out that particular Denso O2 sensor.

So called 'universal' O2 sensors are 'universally' a bad idea [IMHO.]