You would likely need to use a stock ROM to activate the phone on Ting CDMA.
All custom ROMs that I an aware of do not have the dialer codes needed for activation. They also lack the Profile & PRL update menu items that are highly recommended for CDMA.
The PRL (Preferred Roaming List) is the list of towers the phone may use. The Profile needs to be updated whenever there are changes made in the Ting Dashboard Device Settings for the device.
Rooting a stock ROM is OK, but moving to a custom ROM is much riskier, IMHO.