In Griffiths, did you try to find the solution to the Poisson equation for a disk of charge off-axis, i.e. not along the symmetry axis? Or look at the waveguide problem being tackled. Is that anything similar to what you did in Griffiths? What about using Green's function with the appropriate Dirichlet or Neuman boundary conditions?
Yes, you MAY think you're solving an electrostatic problem that looked familiar, but LOOK AGAIN! All the simplified situations that you are accustomed to in undergraduate E&M are no longer adopted! As stated earlier, these are now closer to what you have to deal with in real life! In fact, if you go into accelerator physics, the first thing you'll find out is that, you NEED Jackson's book to survive!
Zz.