Probably just limited slip shudder. My friend's '03 does it and never did anything with it and has had no problems in the 6+ yrs that he has had it. Mine does it a bit too.
2005 supercab 4x4 STX flareside w/ 4.6 V8,
4.56 LS , Bilstein 5100's,
33x12.5x17 Mud Claw MT's
Spectre cold air intake,
Flowmaster 40 cat back duals with 3 1/2" stainless tips