Ok so my rear differential has 10 bolts so it would be 8.8 right and I think The previous owner put in lockers in the back and maybe done something wrong cause when I am driving on the highway I will get some ruff vibration witch I think it is coming from the back well at least it feels that way or could it be the transmission shifting hard it does it for like a few seconds the stops and everything is fine not error codes are showing oh and my truck is a 99 f150 xlt 4.6 2wd

 

U-joints can cause a vibration at certain speeds, along with tire balance.

 

any sighs by looking at the u-joints could tell if they are bad cause I don't think it's the tires cause they are still new tires not even a year old and I have the checked when I rotate them

 

Check the parking brake too, I had mine want to hang up, after using it on a repair, when I hit the road. Shuddered like a paint shaker. Played with the release for awhile to where I felt it was completely freed, havent used it since and havent had any shudder... Which reminds me, I probably need to get that fixed up