What do I do when the diverter plug in the faucet won't stay up to switch to shower?

That diverter plug uses a rubberlike washer to seal the water flow through the spout. Sounds like the washer is not making a good seal so the water flows to lease resistance, the tub. This is difficult to repair while the spout is mounted. You could remove it yourself with a good strap wrench. A regular pipe wrench will cut into the chrome or finish. I almost never try to repair thes, just buy new for about $12.

Please explain what type of diverter's giving you a problem. Is it on the spout or is it part of the valve? If it's the spout, because they're so cheap, simply replace it. If the diverter's in the shower valve give us the brand name of the valve. Regards, Tom