I need help with this bad, I bought a 98 blazer last week and the fuel gauge will show a little below empty, but if you close the door hard or sometimes hit the steering wheel it will go up to full (which the tank is full, just filled it up) I don't want to buy a new sending unit if it could just be a short so please help.

 

Sounds like a sending unit issue

http://www.howstuffworks.com/fuel-gauge1.htm

 

I would like to tell you that you have a bad connection, too just check your wiring from your sending unit to your dash. A daunting task to say the least, but the outcome would be the same more than likely.

These sending units are notorious for failing, if it read a 1\2 tank or something else I would say you may have a point. If you asked everyone in this forum to raise there hand that had a sending unit problem, you would think Hitler just walked in the room. The good news is you can buy just the sending unit, not near as expensive as the pump. The bad news is you still have to drop the tank and pull out the pump. If you are not very careful disconnecting the hoses you will find that you have to replace the pump anyhow.

Good Luck

 

sounds like a sending unit.
I just bought a 2000 blazer, I have the same issue except mine goes from full to empty and empty to full and everywhere in between, bouncing around...
it stops when I'm driving but I never know how much fuel I have, I tried putting in some techron... Now I'm going to wait to see if it fixes it...
Apparently this is a known issue, I heard that if you have a blazer that is either less than 10 years old or less than 100,000 miles Chevy will cover it...
Apparently what happens is, the sending unit gets too much sulfur deposits on it from the gasoline and it messes up the readings from the sending unit.
You can find generic aftermarket sending units for around $111
or you can get brand name parts for around $400
I heard that the generics die a lot though...
I might get a generic one, since I don't have $400 just to put into a sending unit, unless the techron works...
Hope this helps you

 

ive never had problems with aftermarket sending units. Its just been the pumps. Ive had to replace so many of em.