This is pertaining to my truck (87 Chevy 1/2 ton long bed 4wd) rather than one of my other toys but I'm all out of ideas, I'm hoping you guys can come up with some good stuff. Anyways, heres the breakdown on what the truck does and what I've done.

Basically, I can be driving at a steady speed with no vibration/shimmy at all and then it will start vibing/shimming, sometimes worse than other times. It will be smooth, vibe, smooth, vibe with no other changes. Since its not constant, I think I can rule out driveshafts, wheels/tires, and pretty much every rotational mass. Over the past year, I've had a new rear driveshaft made (old one was bent), new front driveshaft made (slip yoke on old one was worn), brand new tires (BFG A/T, balanced), new rear drums (old ones were flaking and out of round), passenger side balljoints and tie rod end, plus new front 3/4 ton springs, sway bar bushings, steering stabilizer, and shocks. The front axle has manual locking hubs, which in either position do not seem to effect the vibration. I've had the rear apart to find excessive backlash but no heavy wear. It makes no noise so for the time being I'm leaving it alone (waiting on funds for new front/rear gears to compensate for the larger tires).
Anyways, I really have no idea what to try or look for next. Everything seems tight. Am I making a correct assumption saying since its not constant I can rule out rotating parts?

The only thing I've yet to do is have a front end alignment done. With these old trucks, I think the only adjustability is in the steering, correct?
Any thoughts? I picked up my new car trailer yesterday, and with it empty, the vibration/shimmy seems to be more pronounced, although this might just have been my mind playing games. I feel it in the wheel and front floorboards which makes me think its the front end.

Any thoughts or input? I'd like to remedy this once and for all .

Thanks guys
Nick

Did you check the front axle u-joints?
If the're tight or worn and sloppy, that will cause a vibration,

Did you check the front axle u-joints?
If the're tight or worn and sloppy, that will cause a vibration,

If he's got the manual hubs, the front axles shouldn't be turning unless the transfer case is locked in 4x4, in which case they would basically be floating. So when the transfer case(which should be a 208C all aluminum) is in 2wd, and the front hubs unlocked, you should be able to turn the front driveshaft by hand, and when you drive it, the front driveshaft shouldn't rotate. If it is, then I would suspect one of the front hubs are hanging up.

You didn't mention if this has an automatic or manual transmission. If it has an automatic, which one is it? I would suspect that it has a 700R4, unless it has been changed. Could it be more like a shudder, like possibly a torque convertor clutch? If it is the torque convertor, for diagnostic purposes, crawl under the truck on the driverside, and unplug the electrical plug on the transmission just above and behind the shift linkage. If that gets rid of the vibration, then either there is a problem with the lockup torque convertor system, or you have a bad miss in the engine.

Thanks guys. Yes the truck has manual hubs up front that both disengage fine. I can rotate the front shaft by hand with them unlocked. Front axle shaft ujoints are tight anyways.

The truck has the original TH400. It doesnt seem to go with engine RPM, rather just speeds above 50.

I had an old jeep that had a nice case of the death wobble but with a leaf spring style suspension I wouldn't think this would be a problem.

Thanks guys, Keep the suggestions coming, I would like to fix this ASAP.