J.D.
I am a SAHM and I love it... I was made to be a SAHM. I quit a HIGH paying job to be with my daughter after she was born. She has never known anything different than Momma being with her all the time.
That being said, you have a routine. Your daughters are used to what you currently do and your family is used to the economic freedom your job provides.
My first thought for you is to start by putting the money you make away. Only take the money out of it that you pay for daycare. Try living that way for a few months while you try to get pregnant. Sometimes getting pregnant isn't as easy as we would like and it may not happen as quickly as you want it to. This also saves for the delivery of your new one when it happens.
Next, take a week or two off where you stay home with your daughter. Everyone else does their normal routine. You run the house as you would if you were to be home with her all the time. It really amazes me at how many working moms think SAHM's don't work. It's work and it is the hardest job I have ever had. I was a developer/dba/project manager in a consulting firm.
I don't think there are that many part time jobs out there easily obtained. This is a hard economy and it may be difficult to find something to fit an ideal situation. You seem to have a great job you are happy with and that is nothing to scoff at.
The best thing you can do for you and your family is to make sure you are a happy momma and that your house is stable. Good luck with your decision!!!