at around 45 mph and then past 60 my rear end starts jumping up and down, pretty violently, and the ride is almost unbearable. I just got new tires, so its not the balancing problem, and i cant figure out whats causing this, need some help on this issue please

 

if it just started and you just got new tires then it has to be the tire

 

my bros lifted f-150 does this but its only when he hits bumps and stuff cause his shocks are blown and i think he said they are to small also

 

Oak is right

Just because they said they blanced them doesn't mean they know what the hell they're doing. Have a friend ride next to you truck and watch which tire(s) bounces for conformation.

I'd take it back and make them do it again

 

well it used to to do the same thing before the new set, but i know previous tires were worn down and unbalanced

 

check the shocks...
and go buy a bag of gravel or 2 and set them in the back over, or behind the axle and see if it dampens it any

 

Sounds like the tires were not balanced correctly. If the truck does it even on the highway it shouldn't have much to do with shocks. I'd take it back and make them re-balance the tires.

 

The new tires could be out of round and balancing will not fix that.

 

what does that mean? and how can i tell?

 

I'd guess he meant something like a flat spot?

 

What kind of tires are they?

The only way to check easily is to put a set of tires/rims on that you know don't cause the problem, and see if it stops.

I'd have a hard time believing that if the tires are new and from a good quality manufacturer that you managed to get an out of round tire, let alone a pair of em.

You could also move your fronts to the back and try it...