Ok guys I know I have a bad sending unit in the gas tank. Iwas wondering if Ican just replace that or do I have to replace the pump also cannot find just the sending unit online. Or does anyone know how to fix a bad one that is flucating all the time.

 

Go to e-bay. Type in "fuel level sensor 2000 blazer"
Make sure to get the appropriate part (2 dr or 4 dr). The pump assemblies are different, so the sensor may be different too (not sure).

I replaced the sensor on my '97. You'll need to drop the tank to do it.
I ended up replacing the whole assembly a month later because I had a problem with the gasket (Oh, make sure you get a new assembly housing gasket too. I bought my sensor from GMpartsdirect, and it came with a new gasket)