OK Guys...I need your help...

For the past few months, my fuel guage on my 99 GMC 1/2 ton truck has been going weird on me...I can fuel up, have a full tank of gas, and the guage will still read empty, and then it will jump to full, and then to 3/4, and back to full, etc.

Sooooo.....

I took my truck to the GMC dealer today, to have it looked at.

After not hearing from them all day (grrrrr...) I finally called them at about 5:30, only to find out that they had just got the truck in...(double grrrrr......)

They determined the problem to be a fuel sending unit (I guess I'm spelling this right...).

They told me it would be about $500 to fix it. ($170-something for the part, and 3 hours labor)

I told them I couldn't afford that, and I would come pick up the truck....
And of course, the shuttle driver had already left for the day, so, I am having to wait for a another ride, so I can pick up my truck...(triple grrrrrr......)

Anyway, sorry about the venting...I'm just extremely frustrated right now...

My question (finally) is this: What exactly does a fuel sending unit do, and if I don't replace it, am I gonna be in trouble?
(i.e., if it isn't fixed, will I not be able to drive my truck, or does it just control the reading on the guage?)

Please help me...

Thank you....
-Misty