Sorry it's been a while since I've posted--summer is by far the busiest time of year for my job.
I really haven't driven the wagon much at all since the initial post of this thread. After the 46 MPG trip, I left the car parked for several weeks. We didn't have anything that we needed it for, and I can't really justify driving it (even just to "test" it) when I can take the Metro and get 80+ MPG.
However, when I went to use it again more recently, I started it up, pulled out of the driveway, and when I hit third gear the car started shuddering pretty violently. I drove it around a bit and found that the shudder/vibration happened in all gears, but that it was worse at lower RPMS and higher loads. This sent me on a quest to figure out what the problem was...
My initial thought was that since I had done all of these drivetrain mods, using junkyard parts, the problem must be driveline related. I knew that I had a small oil leak dripping down where the engine meets the bellhousing, so I though that perhaps some oil had gotten onto the clutch disc and was causing it to slip. When I got the car up on stands, though, I saw that the leak was engine oil, so the trans wasn't leaking, and I saw that it wasn't a RMS leak either. The flywheel and clutch were completely dry, so that ruled out a slipping clutch. I also checked the U-joints and transmission mount while I had the car up, and they were both good.
What threw me off was that there was no check engine light--so I didn't suspect an engine problem, initially. However, what I've learned is that since this car is pre-OBDII, it won't throw a code for a misfire. So after lots of trial, error, and guesswork, I finally figured out that I had a misfire due to a bad spark plug. I replaced the plug and the car now works fine.
I have no idea why the plug went bad, as it was an OEM-type replacement (with a 100,000 mile service life) and it had less than 10,000 miles on it. At least it was a simple to fix once I figured it out.
I'm hoping to get the "towing" axle set up and in the car sometime in the next few weeks. I have several projects to do with our 6X12 trailer, and it will be a good opportunity to test it out.