Need help! I have no idea what the problem is.

I have a a 1995 Ford Taurus SHO 3.2. 130K miles, AXOD automatic transmission. Just had the trans rebuilt about 5K ago. The problem I am having is when I am driving it seems like the transmission doesn't want to shift into 4th gear. When I go about 35 mph the tach reads the engine is turning @ 2000 rpms. And at about 70mph on the highway the tach would read 3000 rpms. This problem only happens some of the time. I notice it when I am on the highway mostly. But when I drive on the highway for a while sometimes it would shift into 4th gear (Overdrive). Almost about 10 miles before it would shift. Also, I think that this problem is affecting my gas miliage as well I get 18 mpg and all I do is mostly highway driving!

I have replaced alot of componets on this car. I replaced the IAC (Idle Air Control valve), MAF (Mass Air Flow) Sensor, Throttle Position Sensor, Intake manifold gasket, Cam Position Sensor, Engine Coolant tempture sensor, Both oxygen sensors, fuel pump, spark plugs, and just got the transmission flushed. I checked for vaccum leaks but couldn't find any.

I hooked up a vaccum gauge to the intake manifold right by the IAC while the engine was warm and the gauge read between 18 & 19 Hg. and it was twitching in between both numbers. I also heard almost like a ticking noise coming from the engine compartment. Almost like an ingition miss. I have no idea if that is directly related to the problem I am having. Can someone please help me???

Thanks
Joe