Hi there,

Driving into school today I noticed a bad shake from the front end. It felt like a reall bad unbalance or like an engine shake as if the engine was missing. I also noted that it only happened in one gear (the final drive, overdrive was on). If I was to step on the gas or shift down a gear with the gear selector i could cruise at the same speed with no shake. It especially got bad if i gave it a little bit more gas at the time (but not enough to shift out of that gear to a lower one).

Importantly I also observed that when accelerating to the affected speed the truck would shift into the final drive at like 2250 rpm but then the truck suddenly drops the rpm to 2000 and that's when the shaking starts. If i give it more gas it continues to shake worst until the gear shifts down, then its fine again.

The truck is an automatic 99 Ford F350 with dual rear wheel (Not 4x4) with the 6.8L V10 gas engine. It starts and runs fine every time. All the tires were just balanced within the last month. I know I have an intermitent speedo sensor failure in the rear diff (it just stops getting a signal and i have to restart the truck). Not sure if this has an affect on it but i didnt notice any problems with it this morning.

I'm not sure if this is related but now that we're getting freezing conditions I feel like my rear diff has to "break out" in the morning. Almost like the ebrake was stck on and just released but I know it isn't and it hasn't been used for ages.

Any help would be appriciated.
Thanks.