After reviewing the above articles, I decided to attempt to clean the intake manifold and EGR pipe to clear the P0401 code on my 1998 RL. The entire process took approximately two days to complete.
Once the EGR pipe was removed, we noticed that there was significant buildup of carbon deposits on the inside walls, but not enough to block total airflow. We cleaned the pipe and debated whether or not to remove the intake manifold, water passage, and throttle body.
After deliberation, we decided to go ahead and remove the intake manifold - and it is a darn good thing that we did! We found that even though the pipe was relatively clear, the EGR plate that connects to the intake manifold was COMPLETELY blocked! The entire EGR cavity on the intake manifold had significant carbon deposits as well. This helps explain why a simple drilling of the EGR Intake Manifold Return only lasts 60k at best!
After cleaning and replacing the intake manifold and EGR pipe, the car accelerated well and idled much smoother. I am no longer seeing excessive smoke coming from the exhaust, and the check engine light cleared on it's own! Even though it is a greater effort, I definitely recommend a thorough cleaning of the entire intake manifold and EGR pipe.
Thanks to all for your contribution in this thread! Best of luck to all of you in your attempts to resolve the P0401!