Hello, I'm new to the forum. I recently purchased a certified pre-owned 2008 XLT supercab (3.0 liter V6). It was my first time purchasing a truck, my previous two vehicles being a Camry and Cherokee.
I expect a truck to have stiff suspension and a bumpy ride, I'm very used to it from owning a Cherokee... but it seems to me that when my speedometer is reading anywhere from 35-45, the Ranger likes to shake. It's not anywhere near what my Cherokee used to do before I upgraded the suspension where it would "shimmy" side to side, the Ranger just seems to jump a little. I'm not sure if it's because I'm not used to having something with a bed (because of the front being slightly heavier than the back) or if there's a suspension, drive train, or tire problem.
The vehicle came with Sigma Supreme TR tires. I've never had them on any vehicle I've owned, but I'm thinking that they could be the problem (square tire maybe?). Since it is a certified Ford truck, everything was replaced that was not up to par with factory parts at the dealership, including doing an alignment and replacing the ball joints.

 

Don't quote me on this, I am in no way an expert, but it could be a transmission issue? Thats just a guess based on experience with my mom's mustang. If it doesn't like the exact speed you're at, it shudders like a son-of-a-gun until you speed up/slow down. Just my guess, someone else will know better than me.

 

Possible broken belt in the tire or slight bent rim?
Could jack up the truck and give the tires a spin or have a buddy follow you,
Just a thought, should take it back to the dealer and have them look at it really.

 

Balance and rotate the tires.