bad vibration

I left work the other night, short ramp fast accel, and when I got on the freeway developed a sudden vibration at about 65-70mph, not steady but on off at about 1 second intervals, almost a shudder.
Pulled the driveline no bad joints, tried high rpm in second and it's still there,so is it coming from the trans or motor? Mounts look good reved it up in gear, no lifting, could the rear mount be broken (replaced last year)? When going from dr to rev sounds like its in the tranny. Also been hearing what sounds like rocks, or pieces of metal hitting underneath at city speeds, could be the new front tires catching rocks, nothing obvious that I could see underneath.
Any help would be greatly appreciated.
Thanks and God Bless
Bruce