VACUME PROBLEM I THINK?

I HAVE 98 FORD RANGER 4X4 4.O MANUAL. FOR SOME REASON WHEN I HAVE THE HEATER ON AND IN PANEL/ FLOOR OPERATION IT WILL WORK CORRECTLY TIL I REACH A SERTAIN RPM THEN THE HEATER WILL SWITCH OVER TO DEFROST OPERATION UNTIL THE MOTOR REACHES A LOW RPM AGAIN. ALSO HAVE NOTICED THAT SOMETIMES IN 4X4 THE HUBS EITHER SLIP OR DONT LOCK IN. THE HUBS ARE RAN OFF THE VACUM AND I ALSO THINK THAT THE HEATER PROB HAS SOMETHING TO DO WITH THE VACUM BECAUSE I WAS MESSING AROUND WITH THE VAC LINES THAT GO TOWARDS HEATER AND HAD A FRIEND INSIDE TELLING ME WHAT WAS HAPPENING AND WITH TRUCK RUNNING AT AN IDLE THE HEATER WORKED PROBERLY BUT WHEN I DISCONNECTED A SERTAIN VAC LINE THE HEATER WOULD SWITCH TO DEFROST SO I THINK SOMEWHERE ALONG THE LINE OR ANY LINE THEIR MIGHT BE A PROBLEM WITH THE VACUM SYSTEM MAYBE NOT ENOUGH VACUME TO HEATER IN HIGHER RPMS OR MAYBE A LEAK IN THE SYSTEM SOMEWHERE AND IF SO COULD THIS BE A CAUSE OFF THE HUBS SLIPPING OR NOT LOCKING, ALSO TOOK THE VACUME LINE FROM THE HUBS ON THE TOP OF MOTOR AND DISCONNECTED IT WITH ENGINE RUNNING AND IT SEEMED TO HAVE GOOD SUCTION AND I COULDNT SEE A PROB BUT STILL COULD BE A BREAK IN THE LINE SOMEWHERE CLOSER TO HUBS BUT POINT IS THEIR IS VACUME TO IT SO ANY HELP WOULD BE APPRECIATED IM NOT TO FAMILIAR WITH TRUCK (JUST GOT IT) thanks