ABS light and problems in 99 Ford Ranger

We own a 99 Ford Ranger with around 80k miles. Yesterday, the ABS light came on, and later in the day was followed by assorted problems. The speedometer is not working below 40 miles an hour, and the overdrive is kicking in and out, causing the vehicle to jerk on the road. I am almost certain the problem will be a sensor, but am uncertain as to which one. I have contacted local auto parts stores and none have a machine to check ABS codes. It's the weekend, and we can't take this vehicle to a mechanic (even if we wanted to spend the extra bucks) and I'm looking for a realiable way to find which sensor needs to be replaced. The weather is poor and doesn't show any signs of letting up soon, and the truck isn't safe to drive on wet roads.