D.M.
He's entering a new phase/stage in his development...this is very likely a form of Seperation Anxiety that he is developing.
You said you wait for the 'sleepy signs' and then put him down...Do you have a routine that is consistent? Or just waiting for the signals? A consistent routine, helps toddlers know what to expect and that the next thing is coming is sleep and not play or food.
Is he taking two naps or just one? At around this age many kids transition from two naps to just one, and this behavior may be due to being 'OVERTIRED' and uncomfortable with 'trying' to get to sleep when he can't. The crying/fussing is him needing your help and you're answering that with the rocking, which is awesome!! I would take a look at sample routines on www.babycenter.com for your son's age and go from there.
When my little dude entered this phase/stage of his development, his Seperation Anxiety only appeared at bedtime and naptime for about a month and then spread to the rest of our day. But, with a few little changes to our day I was able to ease him through it. I too, rocked my son to sleep until he was comfy and then let him sleep on my bed so he could keep smelling my scent and get the comfort...he slept longer. If you want to keep him in the crib, I would get a lovey that he can cuddle with and keep that has picked up your scent. I did this with my son, who co-sleeps and it gave him something to look for when he woke up that wasn't me. I began, talking up the lovey and he even picked it out...I explained how the lovey was his new 'big boy' friend that would keep him company while he slept if Mommy wasn't there. It tooks a few weeks, but he went from looking for me to just grabbing the lovey.
There are SO many things that start happening in this phase. Our kids are more mobile, so they want to be everywhere ALL at once. They don't want to miss a beat and taking a nap or sleeping means cutting out their exploring time. It's not really a rebellion, he's too young to be rebeling just YET...but, he probably will eventually...
Try making him a part of your day and the new baby coming...let him watch while you do dishes. I used to give my son plastic tuperware to dry/wash on his high chair and he loved that. Put his chair where he can see you cook dinner safely or little tasks...make him your helper and let him be a part of the daily routine, when it's safe and comfortable for both of you.
Toddlers benefit A LOT from explanation and conversation. It not only helps with their own speech development, but they understand FAR MORE than many people give them credit.
Best of luck!!