I think that the reason I only fried the taxi output is because I never ran the engine with the tach wire attached. All I did was turn the key on and noticed weirdness in the instrument cluster and immediately turned it off. I suspect that you would get the high voltage spikes only if you run the engine.

I read somewhere on the forums (either here or on volvoforums) that the ECU uses the taxi signal to help smooth the transition to idle while the car is moving. I had noticed the almost dying idle prior to reading this info and that is what clued to me that this problem could be related to my cruise control failure and pursue a fix.

Never under any conditions did I notice any high idle. However, I do try to keep my throttle body nice & clean and the TPS adjusted.