Try raising the P0351 an inch or two and see if the error goes away.
That's a pretty generic OBDII code for being unhappy with a coil. It covers a lot of products and a lot of problems, and not real specifically. My book suggests these causes:
1 Faulty spark plugs or plug wiring
2 Defective ignition coil
3 Failed or failing ECU
4 Open or short in the coil wiring harness
5 Electrical connection that is poor
Not necessarily in that order.
Based on recent history I'll bank on a poor connection/wiring issue. You MIGHT be able to isolate it by unplugging one coil and then the other, checking back with guzzidiag each time to see if the error changes. Otherwise, you strategy seems sound.
Wait -- what was the question again?