differentials? isnt that only on RWD cars.
Your tranny would slip even on straight aways.
What you should do is jump on the highway.
Cruise @ like 55mph, let go of the gas for a sec ( so rpms drop to 1000 ) then mash the peddle....see if it slips then.

Do you hear like a clanky noise when u turn and feel the "tranny slippin"