My condo seems to have good water pressure from faucets, but the shower is very much below average when using hot water. When it is using cold water it works alright (not great, but acceptable). The shower head and tub faucet both reduce pressure as it's turned hotter, so I don't think it is a problem with the shower head.

My water heater has an expansion tank hooked up to the cold (I'm assuming this means incoming) line. There is also a valve right before the expansion tank that will turn but not actually open/close.

Does anyone know why this pressure difference would occur in my shower/tub only and not the rest of the fixtures?

My bet is the shower mixing valve probably has a "scald guard" feature. which is not functioning properly and is restricting the delivery pressure.

Where would the shower mixing valve be? How could I get to it to check if that's the problem (or if i even have one)?

I do belive that Maintenance 6 is right on with this. Most likely either you have hard water or something such as a little piece of solder got into the valve plugging it and preventing the water from running as strong as you like.

If you have a single faucet handle on the shower the anti-scald feature is found inside this faucet.
If you remove the handle on the front and the fancy cover plate that surrounds the faucet you will find a brass cartridge below. Be sure to turn off the water unless you want to take a shower. There is typically two screws that hold this device in. Remove them. Be sure as you remove each part that you set them on a table in the same order as removed otherwise you may get confused when replaceing them.
once the screws are removed this brass device should be able to pull out of the wall. Be careful when doing this. Under this is some round rubber washers that will need to be replaced.
Take this cartridge to your local big box store or local plumbing supply and get a new one. Many folkd attempt to take these apart and clean them but only to find that they failed and have to do this all over again. The part will cost anywhere from @ $20 to $75 depending on the valve you have. Be sure to purchase the rubber washers if they are not included in the kit.
Replace the parts.
There is a black ring on the outside of the cartridge that has little teeth in it. This is the device that prevents the valve from allowing you to turn to much hot water on thus buring yourself while in the shower. It is important that you set the ring to the right spot to prevent this. If you find that the water is to hot when you turn it back on remove the handle and re-set this ring more one way or the other so the hot water on full is not scalding.

awesome answers guys. thanks.

I just repaired one last week that had a lot of rust in the galvanized pipes leading to the mixer valve. I had to take out the cartridge for the mixer valve and removed the seals on both the hot and cold side. I replaced the lines to the mixer with PEX lines and put everything back together after I made sure the new lines were clear. Bingo! Water Pressure that would beat the dirt off of you!