Still chasing a vibration...

[454c20]

This is pertaining to my truck (87 Chevy 1/2 ton long bed 4wd), 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. 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 .