what would cause a pulsing type vibration on the highway at speed, like on 3 seconds off 3 seconds on 3 seconds off 3 seconds, sometimes it does it enough to notice and sometimes barely, its really weird ive never had any vibration like this i na vehicle before and ive had plenty of vehicles with vibrations.

 

It sounds like a harmonics/balance issue.
I've got a similar issue around 45 mph. It seems that my vibration is coming from a worn rubber bushing around the center support bearing on the driveshaft. Everything else is tight and the tires balance nicely.

I can move the driveshaft around just a wee bit by hand but everything else requires a pry bar to create movement.
The CTD can jiggle things better than my two hands so I think I've found my culprit.

 

ill crawl under and check everything this weekend and next week cause im outta school for 10 days.