Is your trans the five speed 722.6? Cause only three or four of the shifter actuations are electronic the rest are mechanical. And there is a bushing in the shifter mechanisim that needs to be replaced every so often - mine was at 50K miles - the bushing in the linkage, accessible from under the car when shot causes a slightly sloppy shfting action, and when really bad a vibration in the shifter.
Not an expensive fix for the bushing.
No vibration at idle in reverse?
Easy to find fault with car on lift, and car put in reverse.