1986 Jeep Wagoneer The Check Engine light comes on when dri

Yes the miileage is correct.45,000. I know. Amazing huh. Lol

I just bought the jeep a week ago.
I had all the dignostics ran on her, the fluids flushed, changed and filled.
Tune up and oil change as well.
A complete inspection done and everything is great they said. BUT. The Check Engine light comes on when driving at a steady 63-65 MPH on the freeway. She doesn't make any wierd noises, no shaking while in motion, no smoke, just purrs along.
Also when I am stopped at a stoplight and she is idling, she does shake a moderate amount. Is this normal for an older style engine? Could it be a faulty reading or is there something else I should be looking for?

Thanks for the help!