Jeep cherokee transmission troubleshoot?

i have a 1997 jeep cherokee with an automatic (4wd) transmission. recently, the jeep suddenly started jolting (which i believe was the transmission slipping) on hills when the vehicle was trying to downshift. the jeep has 135xxx miles on it. i checked the fluid and it had a little more than it was supposed to. Is the slipping and jolting the result of too much fluid or just an old transmission? how much should it cost to fix?