Gas Sending Unit faults -Help!

[taosx5]

I have searched several forums and some have had similar but not exact issues. Here is mine. From what I have read I think I have a sending unit failure.

Wife drives an 03 E53 with 4.4 since new. Now has 150K miles. This car has been pretty good overall and we have had longer than any other we have owned of over 35 vehicles. She is good about filling up at 1/4 tank or as soon as fuel gauge light comes on. Light usually comes on with 50 miles left. (I drive into her to fill up at 1/4 tank and not let the light come on before she fills up but sometimes that is her queue).

So long story short, I get a call that my wife's car broke down about 15 miles from home. She described it went into limp mode and just quit running. I go there with my scan tool and have to jump it to get it started but it won's stay running. Didn't think about it but gas gauge is on 1/4 tank. No error codes what so ever.

We have road side assistance so I have it towed. Car is put on a slide tow truck and dropped off at an independent shop (I say this because car is winched at steep angle meaning fuel in tank is moving around quite a bit). I am thinking alternator went out and is not charging battery and it got so low that it created some electronic issues.

Fast forward to shop test. It is They turn on key and gas gauge is left of E (but no light for low fuel) indicating to the trained eye it is OUT OF GAS. They put 5 gallons gas in and it starts and runs fine, no errors on scan and no other issues. We take car and I am making fun of my wife for running out of gas.

First thing she does is fill the tank. It was on 1/4 tank when she picked up but took 23.3 gallons to fill. Then the gas gauge registers 3/4 tank. We take it on a road trip and miles left shows around 300. We drive about 140 miles and the trip meter range goes back and forth from 220 to 330 as we climb a mtn pass. It changes in 10 mile increments (never done that before). Goes from 300 down to 220 miles at top of pass then back to 300 +/- after we get back to normal elevation. We get to our destination 3 hours later and we have 300+ mile range (this car usually gets about 450 miles per tank). The gauge reads about 2/3 full. We hang out for about 6 hours, temp climbs from about 60 degrees to 79 degrees, we get in car and start it up. Gas gauge then reads 1/3 tank and range is around 210 miles. WITA?

I decide to fill tank and it takes 11.5 gallons to fill and I filled it to the rim. Range goes from 220 to 320 and gauge goes from 1/3 to 3/4 full. We drive back over the same pass we just came and range goes from 320 to 220 then after 140 miles we get home and range is showing just under 300 and gauge is at 2/3.

I have to say since this car was new the computer, gas gauge, and mpg have been super accurate. This issue went from working perfectly to whacko in one fell swoop. I have read many threads and suspect it is a sending unit on one side of the tank (understand there are two).

If I have to take the tank out does it make sense to put in a new fuel pump (over 150K miles)? Do I replace both sending units? Do I try to clean the contacts and see if it clears up? HELP Please!

Is there a good DIY on taking the tank out and checking contacts and possibly cleaning the tank and reinstalling? I do some of my own DIY but is this above average mechanical experience? Thanks in advance for any input.