shudder/vibration at 80 to 100km

hi ive recently been having a shudder/vibration between the speed of 80 and 100 kmh but only at low rpm during cruse such as 1400rpm to 1600rpm if I hit the gas it deceases if I decrease speed it deceases it isn't violent its jut annoying as all hell I can feel it threw the whole truck ive recently replaced my rear pinion seal because it was leaking I tightened the pinion nut to 210 foot pounds as specs say to but now im worried ive messed something up bye not aligning the nut and yoke ive had my pinion nut come loose 2 times on me now even with it tightened to 210lbs im worried also its not harmonized with the drive shaft and may be causing this shudder but keep in mind I do NOT feel this at any other speed ive replaced both u joints and issue is still there I have a 6 inch lift with 37+13.50+18 tires ive also recently replaced the transmission filters and oil with atf+4