Thank you to everyone who has contributed to this thread. I have removed the impeller! :smile: For anyone with a similar chipper and similar problem, here is the solution:
The center bolt that holds the impeller on engages threads on the crankshaft of the engine. (For the record, the bolt is 3/8" in diameter.) The larger threads on the impeller are for removal.
Since the threads on the impeller are larger, that means that screwing in a bolt puts pressure on the crankshaft, forcing the impeller off.
I went to the local hardware store and purchased a long 7/16" bolt to engage the impeller threads. I then tightened it onto the impeller, and it popped off! (Depending on how stuck your impeller is, this may take a lot of effort.) You need to be careful doing this - I bent the flywheel key on my engine.