here's carrier.plist from sprint 4s 5.01
[Only registered and activated users can see links. ]

If you restored to 5.1, you should have a clean carrier.plist, just load prl using rich's prl post and you'll be connecting to cricket. But its pretty useless cause in order to activate, you need to first reset spc to 0s otherwise ota will fail.

What settings are you modifying in carrier.plist?

With above steps (assuming you 0'd out spc), you'll have call and txt. In order to get mms & internet, you have to modify mv settings. To do that, you have to patch commcenter then do some of rich's magic. Rich is doing it all locally on the iphone somehow, maybe with IDA for iphone???

Let us know of your progress....