When were the valves last adjusted? Spark plugs last replaced?
>>> Spark plugs were replaced about 3 months ago when ignition coils were replaced (not sure if they replaced all 6)
Does the shudder come only when cruising down the road at a steady RPM, or does it shudder all the time?
>>> Steady RPM around 1800-2200 RPM
Does it shudder at a specific RPM or vehicle speed range (say between 28 and 40 mph), and does it stop if you take your foot off the gas and then put it back on?
>>> This is what the service guy said to check. I am not the primary driver, so when I drive it, I can test it.
Was any work done to the vehicle recently just before the shudder started?
>>> Not prior. The shuddering eventually caused a CEL on the dash. This is what caused us to get the ignition coils replaced. Cylinder 2 and 4 were not firing.
Unfortunately, I doubt there were two bad coil packs to begin with. If there were, your engine would barely run, check engine light would be flashing and exhaust would smell excessively rich (strong). Idiot mechanics (myself included) replace ignition coil packs because the check engine code notes a specific cylinder it thinks is misfiring, but this is almost never correct.
I have a lot of experience with your engine and engine shuddering. I'd like to help you troubleshoot this.