The coolant was flowing out of the old radiator cap, causing the overflow tank to be empty shortly after filling it each time. The temperature gauge in the truck was giving normal readings though, so I don't believe it was overheating.

Though the truck hasn't been shaking ever since I replaced the radiator cap (about 70 miles ago), I did get a check engine light after about 50 miles. I took it to Pep Boys and they told me it's returning a P1336 code, which from what I can find online has to do with the camshaft position sensor, the crankshaft position sensor, or something mechanical causing one of those sensors to return an error (e.g. the camshaft synchronizer being out of time). Pep Boys cleared the code, and the check engine light came on again after driving for about 15 miles.

As I mentioned in my original post, I replaced the camshaft synchronizer about 1000 miles ago (not the sensor though). When I placed the synchronizer, I was pretty confident that I had put it in the correct position. My procedure was as follows: (1) put the engine at (or at least at what I thought was) TDC, which I took as the point where the "0" degree mark on the harmonic balancer lined up with the little metal triangle/wedge located above (and I believe right-of-center) of the crankshaft position sensor; (2) removed the camshaft position sensor, and then placed the alignment tool on the camshaft synchronizer as a check to make sure it was consistent with what I thought was TDC (it did fit); (3) removed alignment tool; (4) removed old synchronizer; and (5) installed new (and oiled) synchronizer (fitted with alignment cap) into engine. When I placed the synchronizer into the engine, I was off maybe a tooth, and so the synchronizer itself is not in the same position as it originally was (I tried to pull the synchronizer out to redo it, but couldn't get it out). The tab on top of the synchronizer was put back in the same position though (since I used the alignment tool), which I was told is all that matters.

Any thoughts on what the problem could be or what I should next? If I had installed the camshaft synchronizer incorrectly, would it be fine for 1000 miles and then start causing problems after that?

Thanks again.