Epilogue. I've read so many posts about problems that just end without saying what the final answer was, so I wanted to write this so someone searching for a solution in the future might get some insight into what happened. I got the solenoid and a new controller from Sidepower and headed back to the boat to swap them out. The stern thruster is IP protected but if you tell them you have a diesel engine they are willing to sell you the parts. What I expected to be a 4-7 hour job ended up taking 2 and a half days. I removed the engine and thruster batteries for access. I had to remove most of the exhaust hoses to get to where I could remove the waste tank. The connection between the toilet and the waste tank was a real bear to get off and I spent probably about 4 hours just on that one connection. I did not remove the spacer between the doors on the back of the cabinet below the fish box and I couldn't get the fish box out but I could string up the hatch underneath to keep it out of the way. I finally got the waste tank disconnected and found that I had to remove the water-lock and the generator exhaust hose as well to get the tank out. I finally got to the stern thruster and discovered that the tool I bought from Chapman was still too long to get at the allen bolts. Someone mentioned that for the price of the ratcheting allen wrench you can get from sidepower you could buy a whole kit from chapman, so I went that route. I bought a metal saw to make the tool shorter, but it was so cheap it wouldn't cut the tool down to size. So I went back to the store and bought a Dremel tool to use the cut off wheel to cut the tool down to size. I got the stern thruster off and took it apart. There was a fair amount of water inside which would explain a lot of the weirdness going on. I knew the motor itself still worked because of the symptoms. I dried it out as best I could and added silicone seal to the base where it appeared it may have been leaking from. There was a fair amount of water under the thruster and I cleared out the weep hole to prevent that from happening again but that's something to check from time to time. I started disconnecting the leads from the solenoid and one of the leads actually came out of the old one which might explain why it was only pushing in one direction. I put the new parts in and reassembled the unit. Once it came time to re-mount the thruster and reassemble everything else it took about 3 hours to get it all back together. I pulled the boat out of the water to make it easier to do this work and I planned on replacing the thruster and prop zincs anyway, so once all that was done we put the boat back in the water and started it up and tested the thruster; success. Thank god because I probably would have jumped in the harbor at that point if it didn't. So a lot of blood, sweat, and tears went into fixing the thruster but it works fine now. My hands are scraped up and hurting, my forearms and shoulders are bruised up from banging around in tight spaces, and my arms are sore from all the wrenching, but I love having working thrusters.
