Fuel gage or level sender

My 2004 TB has 92K miles. bought used. appears to be well taken care of ..
I recently had problems..
The fuel Gage was working fine or appeard to be working fine..

I filled id up with 50.00 at $2.85 Gallon gauge read about 1/4 maybe a little higher.
I took it on the e-way at about 75 MPH . and when I got home it showed between 1/2 and 3/4 tank.. for about 125 miles of travel.. so added 25.00 about 12 gallons. range showed 450 miles and fuel gage was full.. I have since put about about 50+ miles and it is still showing 3/4 tank..

When the vehicle is started, course the Needle is on E. but as soon as power is added in drops to below E and then climbs up to 3/4. and sits

Disconnect the tank sending unit see if the dash gage responds, although this sounds like a bad sending unit in the tank, which has been somewhat common.

where is this located and is it easy to get to. I tried to get a manual, but the library is closed today

Get under the truck and look for a connector harness near or above the tank.

well I don't think my gauge is bad, I filled her up today and it only took 8.5 gallons as gauge was on 3/4.. maybe in paranoid

All 3 of my blazers did this. It is the sending unit on the fuel pump and the pump will most likely go out within a year of the sending unit giving out. It is not easy to do. You need to drop the fuel tank to get the pump out and i suggest that it is empty b4 you do so.