Mark,
As Tony said
On every regional, lets take a 2wd round, there is the regional 2wd series taking place and also an open class which has to be 4wd. This gives all club drivers the chance to attend and race no matter what car they have. If you have both, then you can do the 2wd, or you enter your 4wd in the open class. Visa versa for a 4wd round.
Tyres is Ballistic or Schumacher Mini Spike, which we all use anyway, so that's no biggie. This is only on the rear driven axle.
Any questions, post up.
Check out Steve's long post in the Mid South forum area (or the same post is on our own Facebook page.