Would a full septic cause your downstairs toilet water level to rise when your washer tub is emptying or could it be a blocked frozen vent?

Yes, your problem may very well be beyond the tank the water is not escaping fast enough out of the tank, to cause the water to rise at the less restrictive point, look beyond the tank make sure the water is coming or going out of the tank unless you have never had the tank pumped out, but in most cases the line beyond the tank is crushed or blocked.