I have an 08 f150 rwd and when I do 60mph or more and there is the slightest bump my rear end wants to slid out on me.
In some cases it is really bad and will freak you out. I checked everything and it all seems tight. Any suggestions?

 

How are your shock absorbers and alignment? Tires and tire pressure?

 

Inspect the rear suspension very carefully. Something is either broken or loose. It could be very dangerous.

 

It almost sounds like your u brackets are loose.

I betcha it is crab walking if you have someone watching you from the back or front.

 

It did it with my old shocks too. Just put new ones on. Put new tires on even different tires and still. Just got aligned again.

 

I'll have to check it out. I know they are tight but I will check again

 

Sounds like the shop that "aligned" it for you doesn't know anything.

There IS something wrong.

I would take your truck to a dedicated spring/frame shop. They WILL be able to fix it, instead of you horsing around with it.

 

You could take some measurements, hub-to-hub, to check if your rear end is the same distance from the fronts (wheelbase) on both sides.

 

Yeah I will definitely do that.

 

I just checked and it's the same front and back. Could this be due to a worn leaf spring or bushing

 

Sure, that's very possible. Take it to a pro if you can't figure it out.