I had the kinda same thing with my 700r4 Auto. Check your transmission mount or the driveshaft may be out of balance. Like an out of round cylinder head.

 

I had a wierd vibration caused by Goodyead GS-C tires with 15000+ miles on them, and I know of another person with similar problems. Just something else to think about. This was high frequency vibration though, like a vibrating chair/recliner.

 

check and see if your missing a spacer between your crossmember!