Simple phase check is to look at each u-joint and make sure that each fixed end of the driveshaft is inline front to back. If you have a two piece driveshaft, then the second piece would have fixed ends 90 degrees off of the first driveshaft and again, inline front to back. Single piece will have the transmission yoke and rear axle yokes inline 90 degrees from the driveshaft fixed ends.
We had a electric 5 ton truck at work with a four section driveline. Two sections had the yokes bolted on out of phase, plus the slip joint out of phase. These were imported in and converted to electric, so a lot hands worked on them. The vibration was so bad, it blew the gear reduction out in the motor assembly, you really didn't notice the vibration since top speed is 52 mph. On a steady flat smooth road, it did just what yours does, smooth then vibrate, smooth then vibrate, etc......
Put it up on good heavy duty jack stands and run it up to speed, a driveline vibration doesn't really need a heavy load on it to vibrate, just rpm.
