1997 Dodge Caravan Shifting Problems with 1997 Dodge Carava

This past summer I was driving the van at around 45 mile an hour and all of a sudden the RPM's went all the way up to 6000 and the van started to sputter. Immediately after the Check engine light came on and I backed off the accelerator and then it wouldn’t shift properly. Anyhow, it its drivable but it will not shift into third and only goes about 40 mile an hour at around 3000 RPM’s. We took it to the shop and they said it was probably a sheered pin ($2000 fix, we only paid $900 for the van)? Then we had a friend hook it up to the code reader and he said something about a 300 code (not sure the exact code.) Not messing with 3rd? Something to that effect… possibly a sensor…any suggestions?