speed sensors?

Hi, My 2000 blazer zr2 has the abs and brake light on. the last time this happened the dealer tracked it down to speed sensors. after a 2 week ordeal because the problem was intermittent, they replaced a few and then blew condensation off the remaining ones. the problem was gone.
well it came back 6 months later. It seems to show up when i tow my 18 foot bassboat more than 50 miles. Can I change these myself? Is this a known problem. any help would be much appreciated thanks. john