If you aren't fighting in front of her (raising your voices, arguing, getting upset), and if not much has changed for her since her dad moved back in (like she wasn't sleeping with you while you were apart and now she's not allowed), it could just be a normal phase--something that would have happened regardless of living arrangements.
I would just stick with one thing (time out, for example) and be consistent--both of you do the same thing. Every single time she behaves a way that is not acceptable, she's in time out. I had a time out area for my son (now 12 years old)--he was so squirmy and wouldn't/couldn't stay in a chair, so the time out area was a bench in the entryway of our house. It was a small, confined area and he would sit on the bench, move to the floor, sit on the stairs, etc. I let him move around, but he had to stay in that area (where there were no toys or anything interesting). One minute for every year old and at the end, ask her why she had to sit there. Give her positive alternatives--for example, don't say, "No throwing toys!" instead, say, "Toys stay on the floor or in your hand." Don't say, "No hitting!.", instead say, "Gentle hands." or "Soft touching only."
And I would have a sitter keep her really busy--if you're gone for two hours, have the sitter keep her very busy--take her outside to blow bubbles, write on the sidewalk with chalk, play with the hose or sprinkler, play in the sandbox, take her for a walk in the stroller, take her inside and play with play-doh, have her help bake cookies, have her sort dried beans, string beads, paint, read books to her, dance to music, sing songs, etc. Just keep her busy. If she's busy and focused, she's less likely to be busy and into things or behaving inappropriately for attention.
All the best to you and your family.
Absolutly as Agiesmom says and make sure the Baby sitters are playing with her , ask them when you come in, with your Daughter there ,what Fun did they all have.