strange transmission problem 96 blazer 4.3
 

so driving home down the highway at a speed of about 55-60mph, my SES light come on and so the next day i take it to autozone and they run my codes and it comes up P1870. i think... either way it was a transmission slippage into 4th gear. i notice that now that around 40mph the truck cant decide whether to go into 4th gear or stay in 3rd. once it gets up to around 45-50 it stays in overdrive. after driving today about 2 hours, all in overdrive on the highway, the SES light goes off. i put some more transmission fluid in because it seemed low. so now that the light is off im wondering if this means that the problem was jut that the fluid level is low, or does this mean that i need to rebuild my transmission... im almost going through mood swings because i get depressed when the light comes on and i get super happy when it shuts off.

hope and pray it doesnt come on again