Most people describe a completely worn out 3-4 clutch as a shift into Neutral, just banging the rev limiter. If it stays in 2nd gear, that indicates the WOT shift table (in the tune) has too high a MPH to make a shift. This could be caused by slippage in the converter, slippage in the trans, or if the tune was just changed - a bad tune. It commonly happens after a higher stall converter is installed.
Was anything changed recently? Is the car stock?
If nothing was changed and the car is stock, it could be a slipping band while in 2nd gear.