99 gmc silverado driveline problem help!!!

[tlbob]

ok so this problem startred about a week ago truck has 65k on it its a 1500 z71 no lift all stock 5.3l 4wd new body style it has a driveline vibration at about 15 to 25 m.p.h i have check a the u joints and they are all good checked all the motor and trans mounts they are all good then i check the drive shaft with a micrometer and it is not out of round so any ideas would be great thanks guys