Alright, 2000 blazer 4 door. The gas gauge shows full all the time, the fuel pump works fine, good pressure. Is this a common problem and if so, do I have to replace the whole fuel pump assembly? Thanks.

 

That is most likely the sending unit that is causing that issue. There is a float inside of the tank that lets the gauge know how much fuel there is.
I would first start by checking the fuel pump ground.

When you are under the truck at the rear, find the fuel pump harness and there should be a ground wire from that harness that is bolted to the rear of the truck, usually in the middle of the frame cross member right before the bumper.

When you find it, take it off, clean the ground point and the bolt and the ground strap really well and put some dialect grease on it and bolt it back up and see if that fixes your problem.

 

Like ritchphotos say"Check the ground".If the ground checks out o.k.?Then it`s the sending unit.You have to pull the pump out if and when you want to even see the sending unit.So if you want to spend the money to have your mechanic pull the tank?You might as well buy a whole new unit.Knowing your luck, you pull the tank,replace just the sending unit..The pump may fail the next day.Rule of thumb..If your in it..Replace everything there while your at it.Ex:If you have a bad upper ball joint on one side?You may as well change both of them.Because the one you didn`t change will eventually fail with time.Then you have to go back in and do the same thing over again.