I'm in the same boat as you and was with my last baby, too. Last time my son was almost 2.5 when my daughter was born, and now my daughter is 3.5. Her behavior has been horrible for the most part. I find it works for me to just try to sympathize with what she's going through and try to see things from her perspective. Spending one on one time with her helps immensely and just trying to maintain that connection with her by doing special little things whenever I can. As far as PPD, I can't say for sure but I know that for me, I feel much sadder about this when I am tired and worn out - last night I was exhausted and Luke had been super fussy for much of the day and I just sat there and cried over the changes I've seen in Annie and whether or not I was still a good mom to her, we used to have this amazing bond and I feel like it's been shattered. I am feeling better about it today after some rest. It also helps her a lot to spend time with friends/family who can give her lots of attention. We've had our babysitter come and hang out with her some as we don't have any family close by. My mom and dad are currently in town and they have spoiled her rotten so she's been in high spirits.
I know I am rambling here, sorry. Just wanted to let you know that I'm feeling the same way and after going through this exact same thing with my son, I am very confident that my daughter will bounce back and we will all adjust to these big changes soon. It's kind of like learning a new group dance. :)