Truck vibrates/shakes when fuel tanks are low on fuel

I bought 99 KW T2000 with 570k miles last year. Truck came with 100 gal tanks but i swap them with 120 gal tanks from 2000 T2000 that own as my part truck. Did that last year. Didnt have vibration 55+ until a months ago. Dont know what happened but in that time i swapped AG400 from my part truck since my 99 came with AG200 and threaded pin bushings were bad. Before i installed AG400 i put new Atro bushings in AG400. Also swapped steer wheels/tires from part truck since my tires was bold. Since then in troubleshooting put neew steer Bridgestone R283a tires and before they were installed they spun just wheels on ballancing machine and they were straight and ballanced them with tires. Tried centramatics, than Tru ballance on steer and drives. Ballance driveshafts, spun drives on jacks and there were round, installed new front leaf springs and bushings. New cab mounts. New shocks on steer, drives and sleeper. Leveling valve on truck. Tried moving tth wheel back and sliding tandems on trailer. Drives and trailer tires are in good condition. Nothing helped. Funny part is if i drive bobtail, there is no vibration. Soon i hook up to my trailer empty and loaded, it start to vibrate. Shocks on my 2016 Wabash dry van are year old. Replaced leveling valve on trailer since it was leaking and adjust hight since it was 1" higher with old leveling valve. Did help but not much. Fund out there is almost no vibration when fuel tanks are full, but once they are bellow 1/2, truck start to shakes. One thing, my 99 T2000 is long frame truck 260" WB. Part truck is 232" WB. Fuel tanks on my long wheel base truck sits behind sleeper, qnx on part truck they were tucked under the sleeper. I cant not move them forward because side shirt brackets are right in front fuel tanks and truck is also 11900 lbs on steers when 80k
So its vibration shakes is more like when steer bounces on the road but they not. Vibration feels on the steering wheel and in the seat so both. So why the hell is vibrates when fuel is bellow 1/2 tanks. Had 04 Volvo with 125 tanks but never vibrate

 

Replaced leveling valve with same original from KW. Also on trl too. I dont mess with china parts

 

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I did tried put in neutral, clutch in, out. Even shut of truck coasting in neutral. It still skahes. Its related to speed only. 55-75 mph