Were you shifting while you were stepping on the gas very hard? It might be your tranny saving you since if you are revving the engine very high as you spin the tires and shift to a lower gear you are more likely to cause tranny damage. The last thing you should be doing going up a steep incline is using overdrive as that allows the engine to go to the highest gear. Going up steep slippery inclines it is better to be light on the gas to prevent wheel slip.