Went to the muffler shop. Guy rattled the exhaust pipe, muffler, and resonator. With the truck running, he found leaks in the resonator weld, and starting tapping it. His analysis was that it was shot, and all the interior had essentially decayed and that we needed to replace it.
We cut it out and tapped some more. It sounded very hollow and was kind of light, so he took that as a verification that it was shot. However, when I shook it, no rattling sound occurred. Put on a new, home-made one, which was a little heavier and did not make a hollow sound. Sounds fine when idling or running at low speed. Have yet to test it in 5th running at high speed and coming off the throttle or engaging clutch, but I explained those symptoms to him, and he seemed convinced that the resonator being shot was causing them.
Comments anyone? Think he knows what he's talking about?
Will report back after I test her at high speed on the highway. BTW, there are no rubber connectors holding the exhaust system in place. Could not find the parts, so the exhaust is Oki-rigged onto the frame using welded rebar. Not a great solution of course, but the only one available at the time. The vibration from the road and years of use/rust probably caused the resonator welds to leak, so it was probably a good idea to replace it anyway. I'm pretty sure it is the original, dating back to 1979.5.
The latest work cost a total of $23 (Philippine peso equivalent) including the new resonator and the labor. Can't complain at that price.
That's it for now. Thanks for all the great advice - G2L