4 wheel drive

What is your question?

The lights work but the front wheels will not pull. In 2 hit at about 55 mph it starts making a loud humming noise when you let off the gas. I replaced a worn gear and spring for the for tension inside the axle tube, but it didn't fix the problem. With all 4 wheels off the ground the front tires pull, but you can stop both of them with your hand and they never start rolling again. The front drive shaft does turn all the time when in 4 wheel. Vacuum is good. Please help