I can't get both tires to light up when turning right but I can when turning left. I'm thinking my diff might be the problem. Are they rebuildable?

 

are you sure you have a posi? my 87 is the same way, i can get both going when i turn left and only one going when i turn right, and ive got an open diff. with the 2.73 gears in it. all stock from the factory. when you spin them going in a straight line do they both go?