you said you have air shocks... IMO air shocks are a bad deal. sure its nice to make your truck look like a stink bug or to remove the baja racer look when carrying loads, but they put severe stress on the shock mounts. shock mounts are not meant to carry load, eventually they will fail and break, making for a scary ride as your truck shifts weight and has no more shock absorbing abilities.

if you really want to change your ride height often, get air bags. they are meant to help with the load and won't have adverse effects to other components. but again IMO, its a toyota truck. if your ride height is that drastically changed you probably shouldn't be carrying that much weight in the first place!

 

I forgot to mention this earlier but also check to make sure your pinion nut hasnt backed off.

 

have you checked the mounts, or wheel bearings?...timing?.....front supension parts?....does it vibrate when setting and engine running?...can u turn off the engine when the vibration occurs just to know that it's not the engine causing the problem....if its coming from the rear jack her up put her on stands and run her up to the vibration speed may then you can pinpoint a front or rear problem just for GODsakes be careful and not hurt your self