ADD uses a vacuum actuator to engage the front differential. The vacuum slides a coupler connecting the front axle which activates the diff and the front drive. There are hoses that connect from the vacuum actuator to a switch then to the manifold. Sometimes the diaphram(s) in the actuator will get torn or develop a hole. Age will certainly contribute that happening. The actuator is up by the front axle likely next the diff. Haven't looked in a T100 before but logically that's were is should be.