tough call. you could start trouble shooting your current trans, but if you're paying someone to do so, and dont solve the problem, you would have been better off just swapping out the trans.

i have rebuilt gm transmissions before, and if it were up to me, i would drop the trans pan and inspect the pan for any debris first, check all the valve body bolts are tight, inspect the solenoids and any seals easily accessible. if nothing obvious is found, you may have to tear into it further, and that means removing the trans. at that point you're better off rebuilding it while its out.