Major problems W 88 Runner Please Help

I have an 88 4runner v6 with an auto transmission. I have been told that I have a problem with my rear differential. How do you tell if this is the correct diagnosis. My truck seems to become a bucking bronco when I am accellarating up a hill. It doesn't happen all the time and Nothing happens when I am coasting. The truck lunges back and forth seeming when The trans wants to shift. It happens more often up a hill at high speeds but it also occurs at lower speeds just not as noticable.My engine sounds fine and seems to be working. I was told to clean out my Mass Air Flow intake and I did. But I still have the same lugging. Please Help me.