1. does your speedometer every go higher than 20 or simply bounce around once it reaches this point?
2. have you had this car a while or recently bought?

a. normally abs sensors in the front wheels will alight once you reach 5mph if there is problem in those sensors. long before you get to 20.

b. if the light remains off until 20 then inspect next the sensor on the axle cover and the connector to it.

c. if this car is a recent purchase remove the axle sensor once you have jacked up the rear end and safely put it on stands. place the transmission in neutral.
with the axle sensor out use a good flashlight to inspect and count the number of steel window panes as someone else slowly turns the rear wheels. this turning will make the window pane like sensor wheel rotate. an abs axle has a 48 pane ring and a non abs has a 12.

d. i have seen first hand this problem IF a non abs axle is swapped into an abs car.

e. if you have had this car a while and this is a new problem you might try swapping in a new abs cluster and if this does not help the only things remaining that could do this are the abs computer and relay under the front pass seat or less likely the the electronics associated withthe abs pump under the hood.