Well, first you need to go talk to your recruiter. The same ones that put you into the Reserves can put you on Active Duty. Your unit also has to be willing to let you go, since you just got back in April I would be willing to bet they may not want to yet. Why spend money training you to do a job that your trying to change already? So they will want their moneys worth from you.
Second issue is getting 19D. I am almost 100% sure you won't get that job. The way it work is there is a list of MOS's that is available to Prior Service (PS) Soldiers, such as yourself, and it dictates what MOS's are available to who. It is based on your rank and your current MOS. Right now E1-E4 the only job you would be able to get is 18X (Special Forces) if you qualify for the pre reqs of that MOS. You can't even come over as a 12R based on the latest message concerning Prior Service.
All I can say is good luck. The message concerning PS changes almost monthly so keep checking, especially with the new fiscal year coming up.