Any time the upstairs tub or shower are in use, the water from there backs up in the down stairs bath tub, any ideas?

Drain is plugged after the downstairs bath. Clean out the main drain line.

You have a blockage downstream from the downstairs bath. This can be snaked from several locations:
1. You can pull the toilet and snake from there.
2. You can remove the tub overflow and snake from there. (see image) **OR**
3. You can snake from the downstairs lavatory roof vent.
Good luck, Tom