I would suspect a bad belt in one or both rear tires,seeing increased pressure amplifies the shaking...are you sure the rear shocks are actually doing anything ?..ones that are seized or have zero resistance can cause or amplify these types of symptoms too..
Oh wait--you said you swapped different tires on it and checked the shocks,and nothing changed...my next guess then,going by what you said about the driveshaft being lengthened in someone's back yard--chances the yokes aren't aligned properly and are out of phase some,or the shaft is warped or bent and wobbling--may not have been cut perfectly straight and centered in a lathe when it was modified..
We had a ramp truck at the junkyard that suffered a warped drive shaft,it got tweaked when we got it stuck,and had to use a forkloader to get it un-stuck..after that the thing shook and had similar symptoms as you described about the frame "kind of bending" and the cab moving...the shaft was more like a banana,than perfectly straight..had to search far and wide to find a suitable replacement too..once it was replaced all the shaking went away..
Oh wait--you said you swapped different tires on it and checked the shocks,and nothing changed...my next guess then,going by what you said about the driveshaft being lengthened in someone's back yard--chances the yokes aren't aligned properly and are out of phase some,or the shaft is warped or bent and wobbling--may not have been cut perfectly straight and centered in a lathe when it was modified..
We had a ramp truck at the junkyard that suffered a warped drive shaft,it got tweaked when we got it stuck,and had to use a forkloader to get it un-stuck..after that the thing shook and had similar symptoms as you described about the frame "kind of bending" and the cab moving...the shaft was more like a banana,than perfectly straight..had to search far and wide to find a suitable replacement too..once it was replaced all the shaking went away..
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