If the vibration is mostly in third gear; that seems to indicate it is related to a certain speed; yet you indicated in an earlier post that it was not,IIRC.
Mopar unibodys have a natural tendency to vibrate at about 55 mph. Almost anything can set it off. And it is a very slow, dreadfully annoying, oscillation; I'm gonna guess at less than 60 cps. It's kindof a rolling woo-woo-woo-woo. By 60/62 it is fading and by 65 it is gone. And it does not change with swapping the rear gear, or the trans or the driveshaft; like I said dreadfully annoying.
I found my tires seemed to set it off, big tires in that day; N50-15s. I have later experienced the exhaust system setting it off as well, and open windows too.
In my cases nothing in the car or chassis was actually seen to vibrate, yet there it was; woo-woo-woo. There was no cure except to not drive 55.
When my current ride (68 Barracuda) started doing it, I finally found a partial cure in changing the pinion angle to a lessor amount.
One time I found an out-of-round tire was exciting it. The tire/wheel was balanced, but the slight "hop" at speed got the body going. New tires solved that one