4 Wheel Drive/SUVs: very loud clicking nosie, flex plate, speedup

Question
i have a 1990 2500 4x4 chevey suburban, whenever i speedup i get this noise,but if i put the car in Neutral it will not make the noise when i push the gas, but i will get the same noise if i back up fast, any idea what it could be?? one of my friends said the it might be the flex plate, could it be anything elts??

thank you!

Answer
It is possible that it could be the flex plate , its just really hard to say without being there to chack it out though , it could be something else have someone listen to it there, I wish I could be of more help, and if I think of anything I will sure let you know ADAM