OK, i had a simmilar problem if not worse. Castor kit costs about $50 maybe cheaper, call up Whiteline they are based in Tulamarine and should help you out heaps. You cannot adjust the castor without the caster kit and its not as complicaded as what you might think. When you have bad caster, it means that one wheel is in front of the other and in your case im asuming its the left behind the right.
The caster kit has 2 bolts and about 20 washers or so, and they just replace the bolt on one the wheels and put washers in to align both wheels the same, but ofcource all of that has to be done at the alignment place since there is no way of knowing axactly how many washers to put in with the naked eye.
But what I would do first is get a print out of the results, which should have the following info: Camber, Toe, Castor and obviously for both front and rear axles. If your caster is more than a degre out then you gota get that kit, but i mean the people that put you onto that alignment machine should tell you what the problems are.
Another posiblity is your Camber out of wack, so your right wheel might be too -negative camber to the left one, and that can make your can go leftish. Just to warn you though, camber kits cost around $200+ due to the fact they are adjustable, and also would need to be fited at same place as castor.
