When the bike is cold the starter works fine. But if I have it off while still hot (like at the gas station) I sometimes get a grinding at starter engagement. Is my solenoid getting ready to fail completely? Can the starter solenoid on a '06 be replaced or does the whole starter have to go?
More likely to be the starter drive clutch slipping. And yes, it is something that can be replaced. Harley sells a kit that includes the one way clutch, output shaft, and spring for about $115 (P/N 31516-94); or for less than half the price you can buy just the one way clutch on various web sites or ebay. Usually all you need is the clutch itself. A complete new starter will run about $430 for the black one, so unless the actual motor itself is shot it's much more cost effective to repair versus replace.
I recently replaced the starter clutch on my '05, which uses the same starter as your '06, and I spent $45 including shipping on ebay for the part. That plus a quart of fluid for the primary (you have to remove the outer primary cover to remove the jackshaft bolt before removing the starter), plus a few hours of my time, and I've got a starter that works better than it did new, hot or cold.
Jerry