For some reason my truck will go up in rpms but it won't be able to shift (this is without the 4x4 on) I have replaced filters done all my fluids even put in some additives that were recommended to me by my local auto parts store. Furthermore, when I try to put it in 4x4 my truck won't shift at all and I have to stick it back in neutral and switch between 4x4 and low to get the vacuum's to seal up right so that I can drive a little further.

is this a stick or automatic? if it's s stick have the clutch checked, if it's an auto matic have a trans guy look at it.