What I mean by the sending unit is the whole module in the fuel tank. The sending unit (aka the pump) the float (fuel level) and filter (if it does not have an external fuel filter, it is in the tank) Replace the whole thing which would be all those things. In the GMT-800s, they were known to go out between 60,000-100,000 miles, and the ones they were replaced with would last just about as long. However, when the pump goes out, it won't always just suddenly go bad. Sometimes it does, but most of the time it will give you a hard starting symptom like you were describing. I had an old 99 GMT-800 that more or less did what yours is doing. I would have to turn it to the on position for a few seconds to prime the pump, then turn it off, then back to on position a few seconds, then off, then on again, then turn to start, and it would start. But even that only worked for awhile before it got worse and I had to have it replaced. The FPR (fuel pressure regulator) were also very prone to leaking and failures on the GMT-800s too.