This has been happening for a while now, but it's starting to get really bad. Lets start off with a few days ago.

I went to the gas station (A Citgo) where gas was $3.69. 9/10gal. I pre-payed $40 which got me slightly over 10 gallons. It was almost completely dry (There was something like half a gallon left overall.)

After putting fuel in, the gauge showed exactly 1/4 of a tank which seemed about right considering how empty it was. After about 1-2 minutes of driving, it dropped to 1/8 and stayed there. Fast forward about 2 hours and my mom used my truck to go get some food. When she got back she said when she got in it was at 1/4 and when she was driving for another 1-2 minutes it dropped to E with the light on. When I left to take one of my friends home (an 86 mile round trip, about 4 gallons worth of fuel) it dropped to below the E, light on, to slightly above with the light off, and repeated this about 2-4 times throughout the whole trip.

It seems the gauge is only accurate when there isn't much gas in the tank. Put more than 4 or 5 gallons in at a time and it goes crazy. I've searched the forums 2 or 3 times and haven't found anything like my problem. I know when it is empty because the light comes on and the gauge stablizes to about how much fuel is actually being used. I've already ran out in rush hour traffic once due to this. Is it the sending unit, gauge (Which I don't think it is because the light still comes on & the range stays accurate to where the gauge is), loose wiring (which I've checked on the gauge cluster), floater getting stuck, or a dirty/ clogged/ bad sensor? I'm at my wits end with it and could literally set it on fire.

Most likely, you'll need a new sender. A piece of foam called a float will float on top of the fuel in the tank, and it has an arm that connects it to a variable resistor, as that arm rises/falls it will change the resistance in the circuit. When that resistance changes, the fuel gauge will read differently. With the fluctuation in fuel readings, it could be a number of things, but the sender is, IMO, most likely your issue. This is also the reason the tank reads differently on different inclines. Good luck!

Edit: Also go get a fire extinguisher, so you don't kill yourself or your runner!

Edit2: It could ALSO be a loose connection in the circuitry to the sender.

I'd also suggest using a scan tool first, to see if any codes pop up.

Does the miles to empty display on the digital readout on the center counsel still work ok. If it is also reading goofy I would say it is the sending unit, if it works normal I would say it is something else.

It usually reads about where the gauge is.

I wonder if it is the resistor? If it is separate then possibly could be that? I don't want to drop the tank without knowing what to look for.

I believe the digital readout is coming from ECM "knowledge," after it senses the float higher, it'll give you a readout based on your normal driving habits. That's why there's such a drastic change (with mine at least, lol) when the battery gets disconnected for an ECM reset.

It'll be on the opposite end of the arm as the float, where it connects. It should be a semicircle shape. You can try using an ohm meter to measure the resistance. Get a hayne's manual and see if it sheds any light on it.

I do not know this for a fact but I was under the impression that there is a removable cover under the rear seat that lets you access the fuel pump without removing the tank.

I have access to the Toyota database so no need for a haynes manual. I'll look for an access plate above the seats and hopefully there will be one. Really don't want to have to drop the tank.

I believe this, too. I'm only replying pertaining to the issue at hand. He may, however, end up having to drop it some point or another.

I think they mostly put those access panels on older ones, but fingers crossed mine has one. Just moved out of our house & I don't think the apartment managers would like 10 gallons of gas in the parking lot very much.

Well, you could drive around and use the fuel ga-- oh wait..

Fast forward now...
Yes, there is an access panel. It doesn't look big enough to get the sending unit or fuel pump out, however.

Now it just sits under E with the light on and range reading 0. I'm trying to source a new sending unit, but I don't want to pay the dealer $103 for it but I did find one for $80 shipped. Its going to get done soon, I've run out 3 more times and had to push it a quarter of a mile to get it out of the road.

I love what you do for me, Toyota.

I've had good luck with cleaning the variable resistor contacts on Audi sending units in a few different cars to get them working again if they were being screwy as the level changed from burning fuel. Sounds like yours is completely unhooked or crapped out though? Curious what it looks like when you get it out.