The first thing you need to do is scan the PCM and make sure the PCM is commanding all gears at the proper time AND that there are no codes setting that will cause the transmission to go into limp mode.
If the transmission goes into limp mode, you may only have 3rd gear operation (along with reverse, neutral, and park - of course).
Simply adding more fluid if it is already full is not going to fix anything. While overfilling will probably not hurt anything, it can cause the axle seals to leak.
-ryan
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OVERKILL IS UNDERRATED
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