I tried another sending unit this morning. Sure enough, both senders read the same on the gauge. At full stop, the gauge was only 3/4, then it would drop drastically. At 3/4 on the sender the gauge was at below 1/2 already. 1/2 on the sender was between E and 1/4. So, I'm of the opinion now that it's for sure the gauge.

That said, I went to Summit earlier and picked up an Auto Meter fuel gauge and plan to install it Friday morning. I'm definitely not spending 3 hrs to rip out my dash and go from there.