Truck vibrates/shakes when fuel tanks are low on fuel

Cab mounts replaced a cew months ago. Front and rear motor mounts replaced last week when i did clutch and tranny swap.
Rear torsion arm is original, its in good shape and bushings are good, had hartd time to track one down since its out of production. Should be here on friday

 

Today i was messing with busted led bar/mudflap hanger, geas up whole truck so i took boobtail for a spin with new Yokohama 702ZL drives. Like i said in the past bobtail rides smooth no vibrations, and yes i floor it to 90 mph still smooth. So its gotta be trailer. Only thing left o trailer is to replace 1 year old hendrickson shock. And boy ill be pissed if that $140 worth of trailer shocks fixes problem
Also around same time vibrations started, i installed AliArc moose bumper. I dont know if that 350lbs will make front springs to bounce or not. But also cront springs and bushing are replaced after moose bumper and after vibrations started. I dont know, im loosing my mind over this.
Maybe its Atro poly bushing that i installed on my AG 400 suspensions. Maybe i should keept Paccar rubber ones
So vibrations maybe are cauused by:
1. Aliarc moos bumper
2. Atro AG400 bushings
3. Trailer shocks

 

No, truck came with virgin Bridgestone 726ELs, a lots of thread left but they were too old since this truck made 30k miles a year at previous owner. So tires were dry cracked. Had two blowouts, one messed up quarter fender and last one light bar/mudflap hanger. Thats why i give up on old tires and installed new Yokohama 703ZL drives last weekend

 

And like i said this isnt shaking vibration, its more bouncing whole truck

 

Installed original all 4 hendrickson shocks on trailer and truck still bouncing. Im thinkin switching back Atro poly Swaybar bushings to stock rubber ones. Or replacing all 4 torque rods or bushings on them. They are original from 99 but truck have low miles so they still in good shape, just old

 

I may swap leveling valve on drives with new one or swap air splitter valve that fills air bags. They are less than 2 year old but with with my luck new parts dont last long as original does