When I adjusted my steering trim so it would run straight in forward, now when I turn in reverse and let go, the wheels go back and forth back and forth. What did I do wrong??????

Quite normal for wheel wobble to happen with trailing axles, and going in reverse. Caster effect.

If there is any slop in the steering, this will magnify the wobble as well as the toe-in/toe-out positioning of the tires. If you think it's excessive wobble going on, then I'd suggest checking the steering linkage, servo saver, and bearings

Thank you very much. I'm new at this and don't know squat!