Weird Shaking going uphill, won't downshift

[NHDieselGirl]

OK, I am not sure if this is the right place for this question, but here it goes.

I have a 2008 6.4L F350 and I was bringing my horse trailer home (as seen in avatar) from VT to NH after service and while going uphill (empty) the truck was hesitant to downshift to maintain speed. It started to shimmy a bit. I had to let off the gas a bit to get it to stop shimmying and downshift. I also tried giving pushing down more, that did not help either. I did not have the pedal to the floor by any means and was only going about 60 - 65mph. I did not have the cruise control on. The truck is 100% stock. I am not sure where to begin and I don't want to call the dealer until I have SOME idea what it might be. I have a hard enough time getting them to take me seriously without bringing my truck in with no clue what might be wrong with it.

Thanks in advance for any ideas!

[DavidPhillips]

I believe the shimmy is the same thing I feel if I take off too hard with my 5er. The rear end bouncing around because of lack of stabalization. I have been considering a bar but I really don't haul up big hills that much and taking off a bit easy on the foot eliminates the problem. I have felt it on a steep pass before.

As far as down shifting I think you just needed to press down more but if the truck was already shimmying it is hard to say if it would have stopped or gotten worse.

Basically the rear end just needs to be stabilized in my opinion.

[racqsixo]

NHDieselGirl,

Are you still getting that shaking going uphill with a trailer? I have an 08 F350 that is completely stock and when I pull my enclosed 16 foot cargo trailer uphill with only a moderate load, it does the same thing. I noticed I don't get this with my 1800 pound camper loaded and pulling the same trailer but I don't know if it is due to the increased load or increased power output required for the load. The dealer couldn't come up with anything on the shake. Do you have suspension air bags on the rear axle?

[DavidPhillips]

I'm sure it's due to the load on the springs. I can fill my water tank on the fifth wheel and keep mine from doing it.