What could cause low water pressure in tub and no water coming out of shower head?
It sounds like one of your valves is not working correctly.  It could be that the handle is slipping or that the valve is clogged.  Does it feel like the handle is turning the valve, or is it slipping? If its slipping you can probably just tighten the screw.

As far as replacing it, you might get lucky and find a replacement valve, but in the event you don't, they make remodel plates which allow you to relatively easily replace a 3 handle setup with a 1 handle faucet.  The remodel plate covers up the area in the wall/tile you'd need to remove to swap it out; you don't have to tear out the entire wall.  If you can get in behind the shower you can probably do it without the remodel plate.