Toyota Repair: Toyota AWD Previa hard turning, viscous coupling, toyota previa

Question
My Toyota Previa AWD (1991) has developed a steering problem that I believe is related to the transfer case viscous coupling.  After driving for a bit, it can be very hard to turn the wheels at slow speeds.  I have been told that the viscous coupling may be locking up.  However, I do not know what to do to fix it.  My tires all match, so that is not the cause.  I recently replaced a bearing in the front drivers side axle, but that did not help, either.

Answer
I have very little experience with AWD previas, I guess there were not many sold in the area I live but it sounds to me there may be a problem with the power steering pump, I don't see how the coupling could have any affect on the steering but I could be wrong, sorry but that's the best answer I have.