Ok, so i've recently started doing backing vocals in my band and i've decided to delve into singing a bit more because it's something to do rather than just playing guitar in covers. Anyway, when i'm singing, i'll be hearing my voice in perfect-pitch with the song (as long as it's a man's voice, otherwise i get nowhere near anything) but then i record it and i'm actually like a 5th lower than the song.

This is alright because the song still sounds right and it works fine for backing vocals but it means that when i get down into the low-end of my range i end up just dropping out because i'm lower than i need to be. Any help? or know why i hear this?

p.s. My ear is great at picking up notes on a guitar so i don't get it.
If you can't the lowest note you go up an octave, and if suddenly jumping an octave sounds bad sing that octave the whole song.
heaps of people do this especially in singing. Either you judt gotta improve and practise, conciously sing a fifth higher than you tihnk is right. Or you up picking up the overtone, and need to go back down to the foundation sound.
^ +1

i was also gonna suggest finding all the songs you can sing pretty well and finding out what keys they're in so you can look and see what keys/notes are easy for you to hit and what you need to work on.
Can you pluck a note on the guitar and then sing that note? If so then practice the vocal melody note for note without backing just pick the melody on the guitar and sing/la/hum each note.

Then play the same melody and sing it a perfect fifth down then in unison again just to hear the difference.

You're just hearing that you're in harmony with the song but what you're not hearing is that you're not in unison. If you practice listening to the difference then you will learn to hear it.

Try harmonizing it in thirds as well just for fun.