Ya, most likely the sending unit. If the top of it or the connection is rusty that would cuase some issues. Dropping the tank is quite the task, but straigforward. If you do it, might as well inspect all the other connections and brakets etc...
I've got 234,00 Km (146,200 miles) on my pickup and 309,00 Km (193,125 miles) on my 4Runner
Details are in my sig. I've decided to use my 4Runner in the winter instead of the pickup. When I get a job, I'm going to spend some money and fix some rust on my pickup (which is currently not visible...