How to disperse salicylic acid in sodium lauryl sulfate solution?
I don't have a lot of experience with salicylic acid as I'm just starting to work with it myself at the moment. However, I find that it disperses better if it is added to the solution at a high temperature (before surfactants) and then homogenised (once thickening agents, if any, have been added).

The other thing which may help is by dissolving in denatured alcohol, although I haven't personally tried this myself. Although it's not a shampoo, I have recently formulated a facial cleanser and as described above I firstly added the thickening agents to the water and then salicylic acid, heated to approximately $70~\mathrm{^\circ C}$ and then homogenised.