Decoding XBee Series 1 API Packet
You are doing something very wrong if you get a large number like 51228675 - the datasheet you linked to says that the ADC output is a 10 bit number right-justified inside of two byte (16 bits).  That means that the maximum value should be 0000 0011 1111 1111b which is equivalent to 1,023 decimal.  So if you get something that large you're including more bits than you should. 

Which bytes do you think are the ADC data in your examples above?