Good thinking, and I did check the tires. Pressure was low on one of the fronts and I inflated everything back to spec (30 on the front and 38 on the back), but no change in behavior.

The shaft rebuild was working out great at first – the vibration only developed a couple months after, but symptoms felt similar to what people describe for an out-of-balance driveshaft.

Unfortunately where I live I've discovered there are no driveshaft shops. There's a big industrial outfit that can do them, but the shaft has to weigh a minimum of 12kg, and I don't think the front is quite that heavy. They usually do shafts for big trucks, I think.

 

I'm now leaning toward this being an engine issue of some kind and not the shaft—possibly bad plugs or an engine mount. I'm replacing the plugs tomorrow.

I put a hose clamp on the shaft to see if it would affect the vibration issue, but nothing really changed. I'm going to try it in a different position tomorrow morning when the car is cooler and it's easier to mess with. My theory is that if this is a balancing issue, putting the unbalanced hose clamp on should either make it better or worse. I put it on top of one of the existing balance weights, thinking that was more likely to produce some effect. If you're wondering why I don't just drop the shaft, put it into diff lock and drive it around to see if it goes away, it's because my shaft is fused to the transfer case and won't come off.

I also revved it in park and there is a vibration that comes and goes between about 1,600 to 2,000 RPM, which corresponds roughly to where I'm getting the vibration on the road. Sitting still it's less pronounced than when traveling, though. It is also rocking back and forth on idle.

I checked the wires to make sure they were going to the right plugs, and I didn't find anything amiss there. My hope is that the plugs are just bad and I don't have to replace any engine mounts. I'm far from ready to rule out the driveshaft, though.

 

Just to be sure, since this is a rebuild, did you mark the shaft and flange and put the shaft back on exactly as it was removed?

 

I did mark it and reinstall as it was before. I actually tried rotating the flange to see if it made any difference. Didn't do anything.

 

Man this thing is frustrating me. I'm pretty sure there's an intermittent misfire that's causing a vibration, but I'm having lots of trouble finding/fixing it. Replacing the plugs didn't do anything. I'm thinking about replacing the wires next to see if that makes a difference. It could also be an injector issue or a vacuum leak, I guess.

 

Get one of these.

Inline Ignition Spark Checker

Plug the cap on the plug and the other end into the wire. Good light means good spark. Bad light means bad spark. If life is good from the coil through the wire, light will be good and the problem is in the plug or cylinder. You really need to try and narrow down a cylinder. For example, I had a misfire that did not throw a code - finally it threw a code as #8 misfire. I hooked this up to #8 and light was bad. That told me the problem was before the plug and cylinder and I changed the wires and coils. Problem solved.

If you get good light on all cylinders, misfire is coming internally and then you can start tracking it down.

If a misfire actually is the problem at all.

 

It was the wires. I replaced them just now (without taking off the upper intake - what a pain in the ***) and the truck feels like a different vehicle. I didn't realize how much power I'd been missing, but it zips around now easily by comparison.

I'm happy to have solved this, but I went about it all wrong. I should have done some diagnostics like jamieb is suggesting above and pinpointed the issue, but I don't have those tools at present. A couple things also threw me off: there were no misfire codes, and I'd recently rebuilt the front driveshaft. The lack of codes was a shame because I'd have easily ID'd the issue if it'd showed me a misfire and which cylinder. The driveshaft was just a red herring.

I've learned a few things from all this, even if I probably spent too much money on new plugs and wires. Having said that, if I'd pinpointed it to a single cylinder misfire, I probably would have replaced all the plugs and wires anyway. I'm calling it a wash for now.