The rear anti-lock light is on because the rear anti-lock system have detected a fault. Fault code(s) have been set in the brake control module. Any code(s) stored must be retrieved and diagnosed. There is no way to tell what is causing the problem without knowing the fault code(s) stored.
there is a single orange wire behind the driver side kick panel, if you ground it you should be able to have the trouble code flashed out through the abs light. count the number of flashes & check that code. if that doesn't work try unplugging the control module under the rear seat to see if that turns the light off.
where is the driver side kick panel located, I was unable to locate the control module under the rear seat.
when I put the truck in reverse the brakes seems to lock up, also the truck is slow to acelorate from a stop position.