Is replacing the fuel sending unit hard to do ?? and is it costly ??

Price would depend on where you live and where the part is purchased. The tank would need to be removed to replace the sending unit and the pump is usually included.
Cost of parts could be over $400 plus labor.

The problem is almost always caused by a defective fuel level sending unit inside the fuel tank. To replace the sending unit you must remove the gas tank from the car and replace the fuel pump module which contains the fuel level sending unit. Repair cost is about $600.00 USD's

No but you need to drop the tank and remove the retaining ring that holds the fuel pump and sending unit in.Replace the pump as well the cost is'nt much difference.

Usually this problem originates from the sending unit, not the gauge. It's unfortunate because the sending unit is located inside the fuel tank. It's even more unfortunate because the sending unit is integrated into the fuel pump assembly, which costs ~$200. Sometimes the sending unit contacts can be cleaned / sanded, but most often, because the labor is about the same cost as a new fuel pump assembly, people opt to just replace it.

Good luck.

Typically, your friend is correct, the sending unit fails the most often. You have to drop the tank to access the sending unit. If your car has fuel injection, be ready for a costly replacement, as the fuel pump is part of the sending unit.... Test the old sender with an ohm meter....

there is a check valve in the tank, part of the fuel sending unit. these go bad and let the fuel run back into the tank, that is why it takes time to start. you can't buy it separate from the sending unit. It will cost around $300.00 to fix it, plus labor if you don't do it. I would live with it till some else on the unit went bad, ie.pump.

your feul sending unit is bad. everybody replaces the whole unit nowadays because it includes a new feul pump as well. that is a good price if it is the pump and sending unit together. our suburban repair shop was $1238. we did it our selves for $300 and it took 8 hours to do

the sending unit is in the tank. jack it up make sure fuels low disconnect lines and filler hose and wireing to pump drop straps lower tank.undo bolt to tank top unlock locking ring.take out sending unit and pump replace and reverse.involved but not that complicated unless your exhaust is in the way.hope that helps

The level sensor in the tank has gone bad. You can buy a fuel sending unit for about $100 (you used to have to replaced the whole tanks for hundreds of dollars, but they had so many issues they started selling the sensors on their own).

You can fix it on your own, but you're going to want the tank to be pretty much empty before hand.

Here is an example of replacing one on a S-10 blazer. Should be similar for your truck:
http://www.ehow.com/how_5306311_change-chevy-blazer-fuel-pump.html

Can't say for sure since you did not include year of your car but more than likely it's the fuel sending unit that requires replacement. Some years you can replace just the level sensor on the fuel sending unit but both repairs involve dropping the gas tank and opening to remove the fuel sending unit.