Fuel guage issues...or sending unit

Stangnet,
For the last month, when I fill the mustang up, everyother time or so, the fuel gauge will go below empty (after filling) and the low fuel light will come on. After driving it for a little while, the gauge will go back to normal and the low fuel light will go back off. I filled up the car last night, and eveything was fine. Drove it home (3 miles away), went to start it up this morning, the fuel gauge was below the empty mark and the low fuel light came on. After driving it for about a mile, the Service Engine Soon light came on too. Got to work, shut it off, restarted it, the fuel gauge is working fine but the Service Engine Soon light is still on. Any ideas?
The car is running fine, so I think I will run over to Checker auto to get it scanned.
Searching on the internet tells me it can either be the in-tank sending unti/pump assembly, or the gauge cluster. I would rather have it be the fuel pump/sending unit because it is cheaper. Although $200 is not cheap. I figure it can't be that hard to drop the tank.
Thanks for any input.
Kevin.

UPDATE!
I did a search and found that a member had a similar issue a couple of months ago. The thread listed a part number of XR3Z-9275-AA for a Ford fuel sending unit. I called Ford to ask about the part, and was told that no, the entire assembly needed to be purchased for $280.00. I asked him to look up this part number and he said, 'oh yeah, you can but THAT part seperatly." Ford price, $121.28 for the sending unit. Got on www.rockauto.com and couldn't find a part listed for the '04, '03 or even the '02 GTs. A fuel sending unit for a '99, '00, and '01 are listed, Motorcraft part number PS93. I called Ford back to see what this was all about. The XR3Z-9275-AA is listed as a Ford sending unit for a '99-'04 mustang GT. The PS93 is listed as a Motorcraft fuel sending unit for a '99-'04 GT. It is infact the same part number just listed either as a Ford part number or a Motorcraft part number. Rockauto.com's price for a genuine Ford Motorcraft fuel sending unit part number PS93, $55.79. Hope this helps anyone who has this problem in the future.
Kevin.
BTW, I took the car to Checker today and it had thrown a P493 code indicating a fuel sending unit issue.