Sounds like two problems. First, you need to service the divertor valve. Probably the best thing is to take out the stem and take it to a plumbing supply house for an exact replacement.
Regardless of the position of the divertor, the water should never leak out the back of it. That leak might be coming from the divertor valve and just showing up there, or, the pipe from the divertor or the fitting has a crack in it. You need to look behind the wall and really see what's going on. If it has been leaking for awhile, you could have some major structural problems back there.