I did some research and found a good writeup about Tracker hubs, 1999 and later only. From this description it's apparent that the front-end parts that rotate while towing are the same parts that rotate when driving in 2WD. Also although manual hubs could be installed you couldn't actually get 4WD without modifying the front diff.
I had a Jeep Cherokee for 18 years that had the same sort of vacuum-locked front end and it failed occasionally, once a bad vacuum tank but usually a bad hose or hose connection.
"This is a great system to avoid lockout hubs, if it was not prone to so many issues. Here is how it works: When you shift into 4WD-Lo or 4WD-Hi, a switch is closed which starts a low pressure air pump (about 5 to 8PSI) located behind the front bumper. This pump has an integral pressure switch, which lights the 4WD dash light when pressure limit of about 5PSI is reached (and held). The pump is connected to an actuator in the front axle housing, and it is this actuator ring that expands under air pressure to engage some teeth in the differential, which locks the differential to the axles, performing the same function as external lock out hubs. The pump should be able to expand the spring loaded actuator ring to engage 4WD within about 2 seconds, and will time out after about 10 seconds, if the pressure switch senses that a pressure of about 5PSI has not been reached in that time. When the tranfer case lever is taken out of 4WD, the switch is opened, which stops the pump and drops the pressure to the actuator and turns off the 4WD light because the pressure switch opens. The spring loaded actuator returns to the default 2WD position, and the differential becomes isolated from the axles."
-jbh-
