stfretts

im new to the forum but not new to working on stuff im a diesel mechanic by trade. i have a 04 fx2 f-150 with a 5.4 i bought it with about 85k on it and now have 108 i do pull a enclosed trl alot which is what bothers me about this problem. when you start off i have a bad vibration it feels like the rear end to me but i dont know for sure i put the truck in neutral and checked the u-joints and they felt fine but ive been hearing that these have problems with u-joints any suggestions? also just changed the fluid in the rear end no metal or flakes or discoloration.

note: the part thats throwing me off is it doesnt do it all the time