Help With Rear Wheel Caster..?

Hey guys, i've noticed this mainly the other day when i broke my tires loose in some 360's. Tread wear....in a bad spot.

I have a sub box, along with 2 12's and 3 amps...all on the box, grand total weight is no joke 58lbs. Ever since i had that in there, i've noticed that my rear wheels are alittle negative in caster, meaning the top of the tire is in towards the body...basically the rear wheels look / \...but not to that extreme. Enough to wear out the edges of each tire and not as a whole. Now i went to firestone, and they said something about shims..? Doesn't my tbird have caster +/- adjustments..? And isnt it relatively easy to do..? $$ etc is the main thing here, how much does that cost..?

BTW, when i take out my sub box, the rear wheels are perfect, just like | |... Is that normal..?

95 4.6L bird btw...

yes, caster is adjustable. Put your stuff in the trunk and have someone who knows what they are doing do a 4 wheel alignment on the car. Some places offer lifetime alignments for about $99.

BTW, oops....i meant camber..not caster. Negative camber is / \ and postive is \ /.. Mine is slightly negative...which i read is better for turning in the rear wheels....ala the rear tire flexing to | when turning.

BTW, my front is a-ok.

Yeah, that's what I meant, too. I didn't catch it. And I don't know of any shop that will just do the rear. You may find one...

I had that same problem after I installed my sub box with 2 12's and only one amp. It also was around 50lbs. (big band-pass box). It somehow only affected the passenger side tire though. I just got new tires on the rear (255/60-15, I wanted some meats) and took it to a good tire store where they do allignment. I dont know if you live in the boonies or something but here in So Cal there are everywhere.