Bob will tell you to clean and apply dielectric grease to the big connector under the driver seat, it is vulnerable to water and corrosion, messing up the VSS signal.
Could be a bad ABS sensor (called the VSS) on the top of the rear axle. Cheap and easy to replace, but it also may drive your speedo / odo, so that may not be the problem.
You're 2wd, so I don't know if it's 2 or 4 wheel ABS. I'm also NOT SURE that your VSS drives the speedo, it may not on the 2wd. I'd change it just because it's cheap, easy and likely to fix it.
Search on "VSS" and "ABS".