I'm not sure what you mean about tranny lock up. Do you mean when the gears engage? And final shift? what tranny light? Arrow up?

Anyways pull your tranny codes off the computer and if you do find out what they are and clear them - Since you have a 96 a shop will have to do that for you unless you have the reader itself.

But before doing that, since you had a tranny flush, I would check your tranny fluid. Even though its flushed doesnt mean (that whoever did it) put the proper amount back. If doing the 12-14 liter flush doesnt mean the same amount came out in the bucket. Check your fluid with the dip stick.
Hope for you thats all it is. Good luck.