I have a 2004 Lincoln Aviator that will not exceed 2300 RPM in any gear. I have replaced the serpentine belt, air filter and fuel filter this week already to attempt to resolve the problem... no change. The vehicle runs very smoothly but will not kick down a gear and simply goes through the motions of upshifting at 2300 RPM's. The car runs fine other than this issue. I suspect a transmission issue but could obviously be wrong. The check engine light does not come on, all other systems including A/C run perfectly. Any ideas?

Check the oil level and oil pressure. The PCM may be limiting rpm, due to low oil pressure. Be sure to check for any codes and pending codes, using a scan tool.