It sounds like it could be understimulation, but you could also work on a focus command with her to teach her to redirect her attention. That could help with the barking, and ignoring you when you try to distract her. Nala gets pretty vocal when she's got extra energy to burn, but she only does it at Elvis, not the neighbor dog, although their dog does bark at us *sigh* that dog would yap all day if they left it outside that long.
Besides being under-stimulated, it sounds to me as if she might be starting to discover her "terrier voice." It's important to remember what terriers have been (and sometimes still are) bred to do. Any quiet (non-barking) terrier that got lost or stuck underground was soon a dead terrier, which is why even today terriers are known barkers.