Always helps when asking questions to give year, model, engine...also doesn't hurt to let us know miles.
I'll assume it's a 90 w/a gas engine based on your nick and sig pic. Most likely cause will be a cracked or pinholed hose attaching the fuel pump to the hardline on the sending unit, inside the tank. Over the years this hose goes bad. I've had to replace several for this exact reason. Of course while you have the bed off or the tank down you probably might as well just do the pump and sock (quality brand only; AC Delco or Delphi) and don't forget the frame rail filter, too.
Not saying this is the ONLY possible cause, just one I've run into more than once. I'd probably wait for others to chime in with suggestions though before tearing into a pump job.
EDIT - after reading your response on another thread I guess I should ask you to clarify. I'm assuming what you mean is the starter is turning the engine at a normal speed, it just takes a long time before it actually fires. Sometimes people use the terms crank/start/etc. in different ways.
Richard