Today, driving to school.. I needed to engage my 4x4, and I did.. It worked fine until I got out onto the street then I got to about 10mph and my truck started to pull right then jerk left, pull right again and kept doing so until I took it out of 4x4.... This is only in 4x4 too... I tried both High and low.. Same thing happened. Any ideas on what it could be?

Truck has 170k mi 5.7L V8 VORTEC 350.

- Thanks,