The "brain" of a 1986 Ford Ranger 4x4 with a 2.9L engine, referring to the Engine Control Module (ECM) or sometimes called the Powertrain Control Module (PCM), is typically located
under the hood, on the driver's side fender apron. It's often mounted to the inner fender well or a bracket nearby. It's a fairly small, rectangular black box.
However, the exact location can vary slightly depending on specific options and factory build variations. A shop manual specific to your truck's year and model would give the most precise location. You could also look for wiring harnesses leading to and from the module—following those should lead you to it.
