Sudden violent shaking from rear end, can't reproduce the problem now?

[ghost30]

Hey all, got a question with a hopefully obvious, simple answer.

I've got a 1995 EB, no idea how many miles, that I have been taking down trails and back roads and such... not necessarily "mudding" per say, but definitely where you couldn't take a Pontiac or something. Over the last winter I replaced all the brake pads, and this summer had to replace the front passenger wheel bearing. Otherwise, haven't really done much to it beyond routine maintenance aside from upgrading the 95 amp alternator to a much better 130 amp.

I took it down a trail a few days ago that I've taken it on countless times (I live in mining country, so lots of hills both on- and off-road) and saw there was a small tree branch that had gotten blown over the trail. It was maybe an inch, inch and a half thick, sitting just a few inches off the ground. I eyeballed it and noticed I could shove it aside with the front end with no damage, so that's what I did.

I was heading home on one of the main back roads with several of the afore-mentioned hills, and suddenly (no idea how fast I was going, but I'd say 50-65 MPH judging from the tach) the rear end started to violently shake, as if someone had just dropped an unbalanced washing machine on spin cycle in my back end. I was afraid it was going to actually throw me off the road, that's how violent it was. I don't believe my tires actually left the ground or lost traction, but it happened so fast I couldn't pinpoint anything out of the ordinary. This normally wouldn't have worried me, but I had my wife and not-quite-1-year-old in the truck with me. Kind of a trek to carry baby back to the house... to say nothing of what would have possibly happened had we rolled.

I noticed also that my sway bar has a bit of play in the driver side, but that, I'm thinking, could have been caused by being pulled by a Jeep to the closest gas pump a day or so prior to this incident. I had fuel in the tank, just not enough to actually start the truck, so we hooked a hefty chain between my frame and the Jeep's trailer hitch, and he happily pulled me into the station. I mention this because it hadn't occurred to me until just then, how heavy my truck actually is. Every time the slack was taken out of the chain, both our vehicles would actually shake like someone just hit us broadside... he didn't have a small Jeep either, it was one of the full-size SUV types. 4-door, tow kit, you name it. We both got knocked around big-time. That may have loosened the sway bar a little bit, I'm not sure though how relevant the sway bar might be in this new situation.

Since the tow and the violent shaking, I've noticed my steering, on my normal "bad" roads, is a lot more... shall I say, springy. My steering wheel has been felt almost vibrating, which it never did before. I had a friend who's a shade-tree mechanic take a look at it, and even tried to reproduce the problem, but he couldn't do it. He even deliberately swerved the vehicle on the road trying to throw it off balance, but the truck refused to duplicate the issue.

My thoughts here are a few.
1) I may have had a bit of mud clinging to one of the rotating components, i.e. the driveshaft, one of the axles, etc, which got stuck and started throwing the vehicle weight around, then finally got unstuck.
2) I may have taken a bit of the small tree with me, possibly caught on the driveshaft or the transmission or something, or wedged in behind the wheel, and it finally shook loose, quite violently.
3) I may have damaged something when I got towed to the gas pump. I believe I hooked the chain up to the frame correctly, but that might have twisted the frame ever so slightly with the tremendous jarring we both received (I was hooked on one side, not center, but he was hooked dead center).
4) I might have hit the trails a few too many times with stock equipment and the truck decided to throw a temper tantrum. Either way, as I said, I haven't been able to duplicate the shaking since.

Thanks for helping me figure this issue out. Any additional info needed ,just let me know and I'll get back to you.

 

[my98nnj]

You turned the steering wheel while the vehicle was off (towed) which introduced air into the power steering system. I'm still amazed that FORD doesn't have a huge warning somewhere in the owners manual for this.

With the engine running, turn your steering wheel lock to lock several times while creeping along. You may have to repeat this a few times to get all the air out. Check the PS fluid level too. That should tighten everything up nicely again.