It has been happening for about 2 months now. But when I travel at around 55-65mph, it feels like something is slipping on the driveshaft the limited slip diff.

It is also a 4X4, and when I went into 4 wheel drive, and then shortly after, put it into 2 wheel drive, the slipping went away for awhile. So it has me guessing that it may be something with the 4X4 part.

It is a 2001 F150, 5.4L 4x4, with 146,000 miles on it. So if anyone has had this problem, please tell me before I have to send it in to be fixed.