Ok well last year i had 4 old tires on my L and i did a long burnout right before i went to get new tires installed. So the truck didnt shake prior to the burnout. After the burnout it shook really bad on the interstate. Then i got 4 new tires installed and got an alignment and it shakes the exact same. 2 months later i rotated the tires and hgt them re balanced and still no luck. btw around 40 mph the truck feels like it goes up and down in the rear so i think the shake is up and down not side to side. I have long bars so did they get tweaked during the burnout causing this vibration? he driveshaft and u joints are good i got under there and it is solid. The tires have correct air pressure. Honestly if i cant fix this i am going to sell the truck because it bothers me that much. One last thing when i put my 2 dirtbikes in the bed it shakes even more...so weight in the bed makes it worse and sometimes it doesnt shake at all...it comes in and out and soemtimes it is all the time. I know this sounds bad but if i cant fix this i gotta sell unfortunately.