Sounds like something dragging or binding up. Have you jacked it up, put it in 4 and locked the hubs, then tried turning the tires by hand? Try this in gear and in neutral, see if the diff is differentiating. If you have a pull like that, you have some binding/dragging, or something that is applying a torque to one of the wheels. Since the problem seems to follow the 4 wheel drive, I'd start there.

Any other shaking or noises? What happened when you tried to go over 30?

Frank.

1994 4runner, 3.0, auto, 4.88's, 31's, BJ spacers, Coil spacers, air shocks, D-ring anchors, 4Crawler F/R swaybar discos.
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