Just got this truck today, while my friend and I were driving it home we noticed a violent shaking while it was in third and fourth gear at around 3K-3.5K RPM's. It was also losing power. Any suggestions as to what it could be? I haven't been able to check it out yet, but I might tomorrow.

It's also the 4.9 Straight six.

 

Automatic or manual?

 

Manual

 

Well things are gonna shake at 3-4k on an I6 if something is broken. I would go an inspect the u-joints on the drive shaft.

Can you still shift into 3rd and 4th normally at lower RPM? Where is the shake coming from? I'd also check the engine mounts ans transmission cross member.

 

It shifts perfectly fine, brand new engine mounts too, haven't checked the U-Joints.
It feels like it loses power and kind of bogs out. It's more like a stop and go kind of shaking.