I was having some vibration from the transmission, when I was reaching a point in low rpm and higher gears. It was as if the D was telling me to shift down. Couldn't figure out why it was vibrating until yesterday. I was under the D, putting some bolts in my side step cross bars, when I noticed a loose nut sitting at the bottom of one of my R380 tranny fram mounts. When I checked the mount bushing, it turns out the bushing was missing it's washer and nut, allowing the tranny to flex from it's mount position abnormally due to clutch torque.
Maybe not your solution, but you should check your transmission and engine mounts, and make sure the bushings aren't shot, and the mount upper and lower nut's are secure and tight.