I have very close to the same problem. I have a 1990 with the AX15 with the Vac disconnect and I think it has a leak. This leak may be allowing the shift fork to move the collar and momentarily engage the front right axle. I plan on doing the posi lock conversion or swap in a non-vac axle.
I don't know if this is close to what you have or not but could be a possibility.
A vacuum leak/poor vacuum/failing motor could prevent the shift fork from fully engaging/disengaging the collar.
From the FSM 1990:
Raise and support the vehicle. Disconnect the vacuum harness from the axle shift motor and connect a vacuum pump to the vacuum shift motor front port.
Apply 15 in. Hg of vacuum to the front port and rotate the right front wheel to fully disengage the outer and intermediate axle shafts (i.e., shift to two-wheel drive operation).
The shift motor should maintain the vacuum applied to the front port for a minimum of 30 seconds. If the motor does not maintain the vacuum, replace it. If the motor maintains the vacuum proceed to the next step.
Disconnect the vacuum pump from the vacuum shift motor front port. Connect the vacuum pump to the vacuum shift motor rear port, cap the port for the indicator lamp switch, and apply 15 in. Hg of vacuum to the rear port.
The shift motor should maintain the vacuum applied to the rear port for a minimum of 30 seconds. If the shift motor does not maintain the vacuum, replace it. It the motor does maintain the vacuum proceed to the next step.
Remove the cap from the port for the indicator lamp switch and determine if vacuum was present at this port. If vacuum was present, the shift motor functions normally. If vacuum was not present, proceed to the next step.
Apply 15 in. Hg vacuum to the rear port of the shift motor. Rotate the right front wheel as necessary and ensure that the outer and intermediate axle shafts are completely engaged. The axles must be completely engaged (i.e., shifted to four wheel drive operation) to open the port for the indicator lamp switch.
Determine if vacuum is present at the port for the indicator lamp switch again. If vacuum was present at the port, the shift motor functions normally. If vacuum was not present at the port, replace the shift motor.
Ok, if at this point it still won't shift correctly you may have a worn shift fork. OEM and replacements are cast and will wear. Posi-Lok 4x4 makes a cast iron fork, about $55.
