While I've never removed the bed on a Titan, I have on my old S-10 before. I imagine that it's similar.
First of all, if you only need to slide it back a bit, you'll only be able to go as far as the bumper will let you. Plus you need to stuff something in between there so that the bumper doesn't scratch your paint. You'd likely only have to remove those four bolts to accomplish this. If that doesn't gain you enough clearance, then you'll have to take the bed off.
I'll assume that you already have the four bed bolts removed:
First, remove the tailgate.
Next, remove the tailights.
Unbolt the screws that hold the fuel filler tube to the bedside. May have to remove the fuel cap as well??
I'm not certain, but I believe there are supports for the bedsides that may need removed as well. They might bolt onto the frame or else they are just attached to the interior part of the bed itself.
Due to the design of the bed and how it curves down around the backside of the cab, I believe it will be necessary to remove the rear bumper as well. You won't be able to just lift the bed straight up and off. This means disconnecting the electrical connection for the license plate lights as well.
The bed should be completely unattached from the truck at this point and everything should be clear.
Good Luck.