accel from a stop shaking
 

hey everyone i have a 2000 f-150 5.4 4x4 with the 3.55 ls in the rear. when i start out from a stop the rear seems like it vibrates harsh or shaking clattering what ever u want to call it. i have replaced dif fluid about 20k ago and not seen water or anything. the truck has just under 100k miles. any thoughts of what it could be? clutch packs gone? i was thinking u joints but i doubt that to be the thing since its only on accel from a stop not all the time. any ideas welcome and 1-10 how hard it would be to do the work if u know what it could be. its getting cold up here in northern mn so im doing the work in an unheated garage ha but still dont want to pay the big bucks:(

no one??? :(