you dont notice any grease in the back wheels like maybe the got one of the bearing to hot and blew a seal causing the bearings to go out?

if not id take it back and have then put all the tires through a road force test. it will tell them if the tire is bad. i had one that was bad from new. but mine shook like hell at about 55-60 mph. they got me a new tire and all was good

 

My truck feels like it bounces at 40-45 range. It feels like where the back of the cab and bed meet is where the flex point is for me at least. I have been unable to pinpoint the cause but so far I have ruled out rear shocks, tire balance, different set of rims and tires. I have not gotten an alignment so that might be my problem... but im waiting on the funds to get it done when I get a new set of tires. Let me know what you find out as I would be interested to try also.

 

Often times tires that have a failing belt will wobble at certain speeds, usually a good tire guy can spot the defect before it blows on the road.
These internal defects are all but impossible for an average person to detect.
Could also be ujoint problems, don't eliminate anything until you have reason to.
Have you inspected your ujoints lately ?

 

It very well could be u-joints in my case considering I am still on the stockers as far as I know because they do not have greese fittings. Maybe ill do the rear set later on this summer.

 

I'd check em sooner rather than later pc, a drive train spinning off is not a pretty thing.
A vibration is a warning, ignore it for too long and eventually the part will fail, and you'll pay the price.
I have a vibration myself from 30 to 35 mph, rides as smooth as can be before, and after, that.
Ujoints are solid so I'm about to upgrade my tires, hopefully that'll be the cure for me.