You can replace the spout on the unit and that should fix it. I am assuming your shower diverter is on the spout.
If the diverter is in the valve then you will need to replace the cartridge in the shower rough valve.
A Master Plumber's Answers to Your Plumbing Questions.

Question: The water in the bath / shower in our guest bathroom has started running very slowly. The water at that basin and in the rest of the house works fine. Any idea where to start?


Here is a link with more information on gurgling toilets.
When I take a shower or run the dishwasher the toilet will gurgle and not flush correctly. Is the drain pluged? If I wait 2 or 3 hours all is good but when water is poured down the drain the noise (gurgle) still is there. No water collects in the tub when I shower and drain takes it away quickly.
Your drain could be partially blocked and the extra running water is hurting the flush. However, my guess is that the toilet is "wet vented" using the drains rather than a separate vented pipe. So, when the dishwasher or the shower runs, the vent for the toilet is being filled with water.
Answer: Yes you are correct there is a special tool called a "tub drain wrench". See the photo above. These wrenches are available at home stores or online.
Am I not supposed to? If I am, is there a special tool I am supposed to use? Any help you can give me would be appreciated. Thanks in advance.
Jeff Jones