Hi,
We were in St. Lucia in 2002. Best "on our own" excursion we've ever done. Went through the "organized tour cabs" and found someone outside who was a private cab, and for $80 pp had an all=day private tour from St. Lucia, to Soufriere, saw the volcano and got awesome video, great picture of the pitons, which was on cruise critic as pix of the week. Saved about $100 per couple and had personal tour for 4 in new A/C van.
St. Lucia is really beautiful. Hope you enjoy!