1999 ram 3500 dually 5.9l 360 4x4. truck shakes at 25 and 50 mph for the most part. what could be causing this? has new tires so i doubt a belt is broken.

 

Is it shaking in the steering wheel or do you feel it in the seat?

First thing I would do is check for an out of balance tire...if it still shimmys and you are feeling it the steering wheel, then I would inspect the ball joints, tie rods, etc...

 

its in the seat

 

Okay, so you are gonna want to inspect the rear of the truck...most likely, the prob is back there...

 

My first guess would be a tire balance issue.

 

Does it have a two piece drive shaft? If so check the hanger/steady bearing for play, you might just as well pull the drive shaft and inspect all the cross joints at the same time. Other wise do as Heyou has suggested check all the tires and rims, also jack up the rear diff and check the rear hub bearings for play, this can be done with the tires on the truck; just use a pry bar under the tires and try to lift up.