So about two months ago I bought a stock 06 fx4 with 90k miles on it. I had the upper ball joints and I believe the lower ball hints replaced with Moog ones. had an alignment done and had the guy who does my alignment install 2.5 inch levelers. Took it out it the hills one night and the next day when I turned it sounded like something was shifting or breaking loose if you turned the steering wheel while stopped or going less than 20 miles an hour. You could hear the thump and feel it in the steering wheel. Sounded like something was pooping out of place. the problem seemed to get more and more consistent and then started going away. since then I have put on about 5k miles and started having a problem where if I'm doing over 40 and hit a bump while turning the steering wheel will be off center to which ever way you were going around the bend. If you jerk the wheel you can get it to straighten back out. Last weekend I was driving home from Arizona and noticed what sounded like my wheels shaking across the pavement while doing around 75 but nothing major. I used to drive a jeep with major alignment issues so I have dealt with death wobble before and I know what kind of sounds it makes. My truck wheels would sound like they were starting the death wobble but didn't actually start. then the other day I was driving doing about 50 and I went over a pothole and all hell broke loose. my truck had one of the worst cases of death wobble I've ever had to deal with. and mind you my jeep had MAJOR alignment issues (loose steering box, loose steering column, bent tie rod arm, worn wheel bearings). Since that drive it has happened a few more times and before I start tearing things apart and buying new parts I was curious if anyone has experienced this symptoms. I tried looking through forums but couldn't find much. I am due for a new alignment as my front tires are showing major wear especially on the outer shoulder.

 

With the problem, why do you keep running at high speeds?
Get under the truck and find out what is coming loose.
Is it the steering wheel shaft connection points?
Is the steering box coming loose from the frame.
You first need some common sense.
Good luck.

 

First off, a must for these trucks are caster/camber adjustment kits like the Moog K100025. Without them every time you go off road, the caster/camber bolts will slip and you'll need to get it realigned.
Second your death wobble. I took my truck to 4 different alignment shops (one was even a dealership) to check my alignment because I was having the same problem after I changed my upper ball joints to a uni-ball. All 4 said my alignment was "dead nuts on to specs" even though my tires were scalloping and wearing bad. After changing the lower ball joints, I suspected that there must be too much toe-in so I marked the placement of the tie rods and started adjusting them in small increments to toe-out a little more. Each time I adjusted them, it got a little better to where the death wobble is gone. Evidently, "specs" has got a lot of toe-in and what happens is the tires are fighting each other, causing them to grip then break free causing the wobble.