woah woah woah
where did we go with all this?
i noticed the OP said the wheels don't "lock up" like they used to? what is a powerslide anyway? are you rippin the ebrake and cutting the wheel? and now only one locks?
or you mean to say you floor it and turn left, and it swings out, but if you do the same but turning right, it just smokes the inside tire?
if you speak of paragraph #2, then you are describing a ford. open diff fords virtually always spin the right rear tire. sometimes our limited slip isn't able to overcome the easiness of just spinning the right rear. so if you put it in a certain situation, you might get an open-diff type result.. it's not a detroit locker right?
so the solution: i doubt there's anything wrong with your rear end. but do what you will; any ford with less than 12 months or 12 thousand miles is eligible for basically whatever you want done, paid for under warranty by ford.
boom tubes will not void a warranty claim on a rear end; however, big gobs of rubber in your rear wheel wells or any other obvious signs of abuse will negate any claims that you may make