I have a 06 dakota with the 3.7v6, 6 speed getrag manual, 8.25 rear end (I believe)

my problem is I have an audible vibration that comes from the rear, that starts around 45 gets worse at 55 and stops around 70 it doesnt do it as bad if it's raining or just recently rained. Acceleration or deceleration has no effect on it. I changed the pinion seal because it was leaking and then it was better but not fixed completely for about a week then it got worse again. Also it seems to make alot of noise when reversing coming from the back end

things I have done, changed u joints and that pinion seal I've also changed the front cv axle, and replaced the front drive shaft.

 

Take the cover off the differential and have a good look at the surfaces of all the gears. Jack it up and turn it over by hand, while watching and listening. My 8.25 has the limited slip, the spyders were badly worn and chipped. Good opportunity to change the oil and clean out the housing while you're there. The other options are the bearings at each end of the rear axle, the bearings on the diff itself, or possibly (and more likely) a dragging brake shoe/broken brake spring. So pull the drums and have a good look.

 

Mine would shake pretty good above 80km/h (45ish). Rebuilt the rear axle, discovered the pinion bearings were allll chewed up. So smooth ever since.