Cylinder misfires only on one (rear) bank?

Hello,
I have a 2000 Toyota Solara SLE V6 (3.0L), approx 130K miles.
Earlier today, the car began to run very rough at idle, and the Check Engine Light came on moments later. I scanned the codes and came up with P300, P301, P303, and P305. These are random cylinder misfire, and misfire on cylinders 1, 3, and 5 respectively.
The problem has replicated itself since, and typically all four codes appear. Cylinder 3 misfire appeared by itself on one occasion, and Cylinders 1 and 5 appeared by themselves at another time. The CEL flashes steadily. All the misfires have been confined to the rear bank.
I have cleaned the MAF, changed the fuel and air filters, and also all three A/F sensors. I have also tested with new plug wires (I have three plug wires that interface with the ignition coils).
I plan to change the plugs just as a baseline, though I did replace them about a year ago.
Finally, I will be testing with new ignition coils tomorrow.
I understand that a bad ignition coil can register multiple cylinder misfires - but since each coil interfaces with both front and rear banks, is this the probable cause (as my codes are restricted to one bank)?
I have also been advised that this problem could be related to the ECM or the fuel rail.
Beyond changing the plugs and ignition coils, I am unaware of how to troubleshoot this further. I am also unaware of what problems these vehicles are typically known for.
Any advice you could provide would be GREATLY appreciated.
Thanks.