Does anyone get a minor vibration when trans upshifts under moderate acceleration. '01 EB expy 5.4 triton w auto4wd. dont know if it could be caused by low fluid level in rear diff. doesn't happen all the time, wont happen under 100% throttle, kinda seems like when im running near the upper end of the torque curve but i wont put more power down. could the front drive be trying to kick in at the shift points an then wants to disengage immediately?