sounds like a broken spring in the 1-2 accumulator. this can be found in the bottom of the trans. take the pan off, and you'll see a roughly triangular shaped 'housing' bolted to the trans case with a few pipes coming out of it. drop this out as an assembly, and take it apart, there are 2 pistons inside it with springs in each one. i would be wiling to bet one of the springs is broken. the springs put tension on the pistons, and as fluid is routed to the piston bores, the spring counteracts the fluid pressure, up to a certain point, then the spring 'gives' and the piston can do it'* job. when the spring is broken/weak, it just lets the piston move as soon as fluid pressure is applied, causing the banging/harsh shift condition.
hope this helps.