Heres the deal. I have a 99 Blazer with 4 wheel drive. The problem I have is... While driving between 50 and 60 mph, my engine light will come on. I can see the tach jumping slightly (maybe 100 revs). It seems that the over-drive is having a hard time kicking in or staying on. It gets better, I can drive like that for a day or sometimes less and the engine light will go off. Also, if I drive for an extended period of time, say 45 minute to an hour, She shifts really hard into second gear and slightly hard into third going up, but shifts smoothly coming back down from speed. I do have an off brand OBD II code reader that gives code 1870, but I lost the book. I took that code number into Autozone and they could not find that code number in their "books". Lastly, the shifting problem will go away if I shut the truck off and restart it. Even if it's just a minute or two. Any one have any ideas? It seem to be more of an electrical problem than mechanical, so hopefully it will be an easier fix. (Note I said easier, not easy!)

 

There are a number of possible causes, I would suggest getting the GM code.
Harsh shift with th check engine light on could be caused by an over oiled aftermarket air filter, TPS out of adjustment, contaminated trans oil, pressure control solenoid, or a faulty coolant temp sensor.