Irs problems

ok.. so i was getting ready to put my nittos on our 04 cobra to take it to the track tonite.. and i noticed the rear left wheel was very loose.. i thought the tire was just loose but the hole rotor and all wobbled around.. could this be the inner rear wheel bearing holding the left axle in place? or could it be the cv joint..it was pretty dark outside and ive had a very long day say i just pulled it back into the garage. im thinking about taking it to the dealer for warranty but i have a 2.8 pulley among a few other mods that could possibly void the warrenty.. i was just wondering if anyone has heard or encountered this problem before and i was wonder how hard it would be to do it myself.. i have plenty or experience.. ive just never messed with this car drivetrain wise. thanks

it was the inner wheel bearing