The M104 3.2 has a variable intake valve mechanism that is controlled by a solenoid. This diagnosis may require a scan for fault codes. I am not really familiar with the mechanism, but I have heard that when it acts up you can have funny behavior from the engine. When you say pretty high RPM on the highway, what RPM at what MPH. You may be experiencing a lack of high gear from the tranny..
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.