I got a 98 expedition XLT 4.6L, everytime I engaged the 4X4 or AWD when I'm driving I hear this thumpping/popping noise comming from the rear. but when I take it out of 4X4 or AWD, It drives normal. I dont know what is making that noise. Have anyone ever had this problem. Also everytime I hit 70mph my front end shakes really bad. I got new tires on the front and had them balance. My rear tires are kinda old and need replace. So any help I would appreciate.

 

The mismatched tires are causing the popping noise. With the rears being worn and the fronts not, they have different diameters. Being such they do not rotate the same number of revolutions per mile. This causes binding within the 4x4 system and the tires actually have to slide at some point to compensate.

The high speed shake could just be worn u-joints on the driveshafts.

 

x's 2 for Alloro.

The shake happens when out of 4wd as well? If so, Id start by getting the balance confirmed. Check for bad ball joints. Maybe even check to make sure there are no vacuum leaks that are allowing the hubs to stay locked, which would expose the worn u-joints on the front driveshaft or worn CV shafts when out of 4wd.

 

He wrote, "when I take it out of 4X4 or AWD, It drives normal"

 

He also wrote, "Also everytime I hit 70mph my front end shakes really bad"