Bit of a problem but get a helper and remove the remote shift cable at the engine. Go out and run the toon as you are currently doing and have your helper or let them drive and you do it, shift and see if it works under load. If it does, chase your remote control box. If it doesn't drain your lower unit oil and tell us what comes out including any water, oil color....deep blue-black, or yellow or brown tint. Will be some bubbles since you have been running it. Might put the oil in a clear glass or plastic container that is small and tall to get some height to the oil. After a day or two, if it has water in it the water will start settling to the bottom and you will see it as bubbles also....once the air bubbles from agitation have dispersed. If it has a magnet on the drain plug, the amount of metal deposits clinging to that too.