starts revving, like it didn't want to go into the next gear?

Hi, thanks for any help you can spare. I bought a 2002 Durango,no warranty. Looks and, DID, run great. I've had it for about 3 months now,and the Check Eng. light came on,then went off a day or so after that. A few days later I took it in to a local Firestone service center to get 4 new tires and an oil change. After the new tires & oil change,I drove down the road,and it starts revving,like it didn't want to go into the next gear.Turned around ,went right back,and they tried to put a scanner on it to see what the problem was. To all of our surprise,the diagnostic connector,(ALDL), was half-way pulled apart. Not able to hook a diagnostic scanner up to it, they told me they would try to order a part for it,but it should be o.k. to drive.-Now I can only drive it up to about 45 MPH before it starts revving up to 5,000+ RPM's.Seems like it just stays in 1st gear.- Is this a sensor,or am I about to lose a transmission? Please help.