91RANGERNJ

picked up a 91 xlt 4x4 4.0 automatic,guy says needs transfer case motor.i dont think so.i believe it may be vacuum modulator.truck does not engage reverse or neutral i feel it go into reverse very slight kick but does not move same as neutral feel disengage reverse but it does not roll at all in neutral.thought might be rear brakes(jacked up rear front wheels are free)disconnected driveshaft at rear rear wheels move freely.drives forward but feels like in a high gear .not really sure exact function of modulator but does anyone think this could be cause?also i didnt change fluid or filter yet possible clogged filter?i am mechanically inclined but never really did any trans work.so i am a little at a lose.thanks