I just finished Act 2 and I'm having a similar struggle with Jaheira.
Based on what we see in-game (panther wildshape) before she joins us, she has at least 6 levels of moon druid. She also used dual scimitars which leans towards some levels in a martial class.
I currently have her with 2 lvls in fighter, but I could see her with levels in ranger with the 2 weapon fighting style. I also think a few levels in Ancients Paladin would fit her, but I'd drop two weapon fighting for dueling or defense.
