I'm not familiar with Scott Klement's JDBC driver, and I'm a little confused by the implication that his (or any custom) driver is in jt400.jar. Regardless, the thing to note is that you are getting a No *ClassDef* Found error, not a ClassNotFoundException.
The javadocs for NoClassDefFoundError say: "The searched-for class definition existed when the currently executing
class was compiled, but the definition can no longer be found. " I'll refer you to:
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