Hi,
Welcome to the Javaranch!
Take a look at
http://www.sun.com/training/catalog/courses/CX-310-252A.xml. There you can read the following:
Prerequisites: Must be Sun Certified Programmer for the Java Platform (any edition)
So yes, you have to be a SCJP before you can try to get the SCJD. But if you have Java-experience and you get a good book (Kathy Sierra & Bert Bates one for example, that's the one i used for both my scjp-certs), the SCJP is an "easy" to get certification, the SCJD is a whole lot harder
Good luck!
Kind regards,
Roel