Tach revs up 300 to 800 rpm on its owne.at hiway speed.

My 1998 5.2 Dakota did the same thing... I removed the Idle Air Control valve and cleaned it with carb cleaner and my problem has been fixed (FOR FREE). At the same time I removed the Throttle Position Sensor and inspected it. I have read that running a very good fuel system cleaner in a tank of premium gas will help clean the carbon buildup on the sensors and throttle body. These two sensors are located on the Throttle Body and remove by unscrewing two Torx screws... the entire process took me less than 5 minutes. I know it is called an IDLE Air Control Valve (or solenoid) but my truck also did it while going down the road at certain speeds.

How to Clean the IAC Solenoid:
This is recommended at every Tune-Up to prevent excess carbon buildup on the IAC plunger. In some cases, you'll notice a slightly erratic idle quality. This is the first step in taking care of that problem. IAC = Intake Air Control (Solenoid).

• Remove the Air Hat from the Throttle Body
  
• Disconnect the sensor harness from the IAC Solenoid (Rear of Throttle Body)
  
• Remove the two Torx-25 Screws
  
• Remove the IAC Solenoid (Be careful not to lose the rubber O-Ring)
  
• Spray some Carb/TB Cleaner in the IAC port on the Throttle Body and let it sit
  
• Spray some Carb/TB Cleaner on the tip of the IAC Solenoid and wipe clean (Do NOT forcefully twist or push the plunger - You will damage the Solenoid. Wipe very gently.)
  
• Spray a little bit more Carb/TB Cleaner in the IAC port on the Throttle Body and wipe clean with a thin/lint-free rag
  
• Reinstall the IAC Solenoid (Make sure you don't lose the rubber gasket on the solenoid)
  
• Reinstall the sensor connector and airhat
  
• Start the engine and let idle for about 1 minute.
  

I would put a throttle posistion sensor on!