2003 Mitsubishi Magna • 308,822 miles

My mitsubishi TJ Executive series II has recently started reving when excelerating, instead of remaining around 1500 RPM it shoots up to 3000 RPM which its never done before, have recently had the timing belts replaced at 300000 along with the spark plugs. I was wondering if it may be the transmission and whether its a sealed unit or am I able to gain easy acess from the top to top the transmission fluid up, I'm the second owner of the car and am not sure of the transmissions history, whats the easiest way to sort this out as the car has been an exceptionally reliable car that has been a true workhorse for the 5 years I've had it.
February 1, 2013.

Check to see if you have a dipstick, there will be two if not sealed one for oil awhich may be yellow and another for trans. But have a transguy scan for codes.

Feb 1, 2013.