Any ideas what this could be??

[Chboro]

I'm the second owner. Had no maintenance records when I purchased 7 years ago. I'm thinking the mounts are all original. When I replaced the plugs and tubes I did examine the coil packs and all were good. First time I ever experienced this vibration at any speed. When it happened I was going a steady 82mph. No excessive play at the shifter. I'll look at getting the tranny and engine mounts replaced 1st.