Try pulling the Fuel Injection Pressure Regulator vaccum line, it should be dry, if you see some gas its bad, that could be the problem, my brother in laws 2001 lesabre has the same problem, and after doing a tune up and lots of other things, we finally figured it was the Fuel Injection Pressure Regulator was leaking
UPDATE: I replaced the ICM and coils. ICM is by BDW and the coils are BDW select. This fixed the highway stuttering "vibration" issue and netted 8MPG. I took a 100mi round trip shakedown drive to Palm Springs after the instillation and noticed the average MPG for the trip reading 29.6. That was with a headwind of the rainstorm moving in and some hills.