With the trans being a Muncie, the reverse gear is in the tailshaft housing, or extension housing, they are called the same. This means the rev shift fork will come with the housing. You can remove it from the car, support the eng/trans at the eng oil pan with a block of wood to protect the pan.
Use the very bottom bolt from the ext housing, this will serve as a drain plug for draining the oil. Change the bushing.
But, before I'd do that, I'd support the rear axle on jack stands and chock the front wheels to prevent movement of the car.
I'd run the car through the gears to see if one could simulate the speed/vibration. With car idling in 4th gear, check the rotation straightness of driveshaft and other related parts for even straight turning. If vibration is still there, eliminate rotating items one-by-one. With wheels off, be sure to install all lug nuts to keep drums/rotors from flying off the axle.
I'd check angularity of driveshaft, it may be off.