The reverse bands are made of cast iron and typically don't fail; the usual failure point on the 1-2 and 3-4 bands is the welded tang; their servo rods break the tang off the rolled steel band. But it is possible that the band itself broke, but not likely. Look at the images on google images for 5R55E reverse band. One of the images shows all 3 bands, and the reverse band is notable for being really heavy duty (cast iron), while the other 2 are thin steel with a clamp tang welded on the end; they don't take as tough a beating when they activate, since they are not changing the direction of any drive member. Anyway, the advice to put a phillips screwdriver up the rod channel is good; push it up and you can feel the band when the screwdriver hits the top of the bearing hole, then about 1/4 more up it will hit the fully clamped position. And yes check those O-rings. If you are having a problem with the transmission going into reverse, you're on the right track. If you still have probs after that, you may have a bad Electronic fluid Pressure Control solenoid. When they go bad, reverse and 1st typically stop working smoothly, if at all, while 1-2 and 3-4 really flare.
