Well.... I feel stupid now. After Mike mentioned an exhaust can make it similar to a flex plate, I looked over and over countless times to make sure it wasn't. Until I changed my oil yesterday, I found the culprit. The exhaust was rubbing against the heat shield on the floor board between the transmission and transfer case. The whole time I looked for it was the truck parked on an incline. Found the issue when it was parked on a level ground. My motor mount may be more worn out than I thought to cause that vibration noise. I'll still add the flex plate in when its time for a transmission rebuild... If I ever get to it.