small vibration when taking off from a stop while turning left

Hey guys,

07 crewcab sport, 202k. 4.0 Has had the following this year....

New Frame
New Drive shafts
New Ujoints
New A Arms
OME 887 springs and 5100's with AAL in rear.

Only when turning to the left from a dead stop, I get a vibration that starts as soon as I hit the gas and lasts for the first 3 seconds of 1st gear. It does not do it straight or while turning right. Its been doing it the whole time Ive had it (one year.) While it was in at the shop for the frame I asked them to take a look at it and they said that they couldnt really find a reason for it except that while it was on the lift it seems that while turning the rear wheels while in neutral, there seemed to be a stiff spot in the rear diff. I came home and changed the rear end grease and used a bottle of the diff conditioner but still have the same issue. If I turn the wheels left and take off hard on pavement the left rear acts like it would if it was in 4wd, kinda dog hops. Is there anyone that may know what this is? Ive looked everywhere, even thought about buying another rear end.

Thanks!
chris