Even though you're on an Alltel plan, you should still be running a Verizon PRL by default. I don't know the answer to your network extender problem, but I routinely switch between 3 PRLs on my phone when I run into poor coverage.
This Alltel PRL still allows Sprint data roaming. If you're on an Alltel plan, it could possibly help if Sprint's signal is stronger at your house.
http://corolada.com/prl/alltel/40042.prl
In the past, 3xxxx series PRLs were designed for dumb phones and 4xxxxx series were for certain smart phones and datacards. This is the latest Alltel smartphone PRL.
http://corolada.com/prl/alltel/40052.prl
Then you have the Verizon Hybrid PRL. I'm not sure why the rep didn't offer to push this to your phone. It allows you access to both the Verizon and Alltel networks.
http://corolada.com/prl/verizon/65036.prl
I'd try all three and see which one offers you the better signal and fastest data speeds. Having a red phone on a blue plan in a converged area can be somewhat of a pain.