AEM makes a controller that will operate a DCDD (or whatever Yota calls their version) center diff., a speed sensor on each output is all you need for sensors in a basic system.
The 99 and earlier NV136 is a viscous coupling center diff, all the controller does is manage the "shift on the fly" function.
Quote:
Pre 99 models
Quote from Truck Trends
Feature a full-time viscous-coupled transfer case that will automatically transfers torque where it's needed. The viscous coupling acts as a torque-biasing device attached to the front- and rear driveshafts in a lightweight magnesium housing mounted directly to the automatic transmission. One set of the discs turns with the front axle, the other set with the rear axle. The discs are surrounded by a high-viscosity silicone fluid that, when deformed, transfers power from the faster rotating discs (or the slipping wheels) to those rotating more slowly (the wheels with grip). A 35-65-percent front-to-rear power split is used under normal operating conditions, while up to 100 percent of the engine's torque can be transferred to the wheels with traction when needed. Road surfaces needn't be slippery for the AWD system to deliver a traction advantage. If climbing a hill or steep grade, for example, the differential will send more torque to the rear wheels to compensate for the extra weight. The AWD also accommodates for speed differences that naturally occur when a vehicle travels around a corner on dry pavement and adds superior cornering capabilities under all circumstances. The system also provides better durability than a rear-drive-only system because torque is continually shared between the front and rear axles.
Packaging is the big problem, the xfer case usually will end up trying to take up space the passenger seat needs.