first off try cleaning the mixer body using a good carb cleaner, they do get gummed up on the inside and this does cause alot of "runtime" issues such as idle and intermittant warning light.

secondly, there are only 2 ways to access the computer on this model truck, 1) use the pc version software and special cable cat offers to diagnose the pre-tier1 and tier1 trucks with the CARB compliant systems.
2) the handheld HD1 diagnostic monitor

OBDII is useless in this case.