Check engine light.

[Chevy623]

My Blazer drives fine all over town but when i reach highways speeds the check engine light comes on and stays on untill you turn the truck off. when you turn it back on it is no longer on. i can't tell you how fast i am going when this happens because the speedo is currently non-op. the tranny does feel like it is not shifting properly at these speeds. it is a loud truck and when it comes time to shift, it gets really baffled sounding, and then loud again when it shifts. it is a 1991 with a 350. i could really use any ideas as to what the problem is. thank you.