The Bartlet administration is getting a shake-up -- and we're not talking about the departure of ''West Wing'' creator Aaron Sorkin. ''Roseanne'''s John Goodman joins the cast of the NBC drama May 14 as a Republican speaker of the House. Now that VP John Hoynes (Tim Matheson) has resigned, will Goodman become Bartlet's wing man? Allison Janney hints that the situation is ''pretty monumental'' -- something that could ''happen in our government but has never happened. It's going to have amazing repercussions for next season.'' Adds costar John Spencer, ''He's going to become quite involved in the Bartlet administration.'' Anything that lifts ''Wing'' is good news for NBC, which has seen the Emmy winner's ratings slide 22 percent this season. ''It's been an interesting year at 'The West Wing,''' admits entertainment president Jeff Zucker. ''I think if you look at the final four episodes this season, they may be the four best in the history of the show. The storytelling and character development...is fantastic.