Hey 64...
You didn't say if it occurs in 2 wheel drive or when in 4 wheel drive, what speed it starts to happen, etc... Did problem just start or???
Could be as simple as tire/wheel out of balance, bent rim, worn-out tire, loose lug nuts, tie rod ends loose, bent, broken, or something like a bearing inside one of the hubs. Check to make sure that both locking hubs (if you have them) are disengaged when in 2Hi. Also check to make sure the axle "u" bolts are tight. While your under the truck check the motor mounts and all the u-joints on the driveshafts. Check the steering box bolts and the frame for cracks where the box bolts up. Check everything out that you can and let us know what you find.