My 98 blazer recently started shifting in and out of overdrive below 60mph. It seems like it doesn't know if it wants to be in that gear or not. Doesn't feel like slipping, fluid is ful and good color, no burnt smell or anything out of the ordinary. Check engine light has come on twice in the last month, but it went out, and is currently still out. Above 60mph everything is fine, no strange shifting, nothing. what should I check? If I had to guess i would think it is some sort of sensor issue or something.