I have just one of my 7 sprinkler zones that seems to be at half pressure when operated with my controller. However, if I manually turn on the zone at the valve, it turns on with full pressure. Does this mean I have a bad solenoid?
Most likely a bad solenoid. Most sprinkler system control valves are built in such a way that repairable/replaceable parts are accessible without having to replace the entire valve. After taking it apart (with the water turned off of course), what's usually left in the ground is just a plastic form with no moving parts and nothing to wear out. That's good because it's a total PITA to swap out an entire valve. I've had to replace many solenoids in my system and an occasional diaphragm. Most of my valves are Weathermatics and are easily repaired. Take a pic of your valve and if possible, the model number and we can better help.
It would be likely one Solenoid valve is not operating properly, or just clogged.
Open the vent screw on that solenoid and let the water out (flush) for a while.
Replace the solenoid, or perform simple surgery, by opening it (with Philips screwdriver) and cleaning it, while checking that the big membrane (about 2 inches round rubber) is intact.
If it partially turns on the solenoid is probably OK something is probably obstructing the water flow. Most of those valves are pilot operated and it does not take much to partially obstruct the pilot part. I suggest you follow knowitall suggestion. You should also check the wires to be sure you have a good connection. Swapping the wires between two valves and if the problem changes it is in the wiring if not the valve is probably partially plugged. If it is the wiring be sure it is also good at the controllers end and any possible splices in the line. Do not forget to place the wiring back to its original connections.