Assuming you have a spin-on freewheel, and not a freehub/cassette system, then only the freewheel can cause the problems you describe. That doesn't assure that a soak will cure it.


If you also need a new chain, replacing both together makes sense.

But if not, a quart on mineral spirits is much less expensive than a freewheel, so I'd give that a go. When you're done with it, the solvent can be put into a pickle jar and stored for future use. in the meantime the dire will have settled to the bottom ans the solvent can be poured off and used again.