Need help diagnosing a fuel gauge problem

I have a 2007 International 9400i. My fuel gauge is currently pegged past the max level no matter how much fuel is actually in the tanks. Previously, whenever I'd fill up the tanks, my fuel gauge would jump around from max level on the gauge to just past it. So I replaced the gauge, and it did not fix the jumping problem.

With my current issue, I'm thinking it's the sending unit. I'm guessing it's either stuck, or finally crapped the bed (truck has 1.35m miles on it). Or maybe something wrong with the wiring or the connector...?

If it is the sending unit, how hard is it to replace on the 9400i? Would I need to drop the tanks or lift the cab a bit to get it out?

 

Got caught up with work and kind of forgot about this thread... Anyway, I did check the sender - I unplugged it while truck was running, and the gauge is still pegged past max. So if it's not the sender, lets assume it's not the gauge it self either, how can I check the wiring? I mean what exactly do I check? I have a multimeter.