Cook the rice according to package directions.
Combine the sour cream and 2 tablespoons lime juice, then fold into the rice along with the scallions and cilantro.
While the rice is cooking, heat the oil in a large skillet over medium heat.
Add the poblanos, season with ¼ teaspoon each salt and pepper and cook, stirring occasionally, until beginning to soften, 5 to 6 minutes.
Season the shrimp with ¼ teaspoon each salt and pepper, add to the skillet along with the garlic and cook for 2 minutes.
Add the corn and cook, tossing twice, until the shrimp are opaque throughout, about 2 minutes more; toss with the remaining tablespoon lime juice.
Serve over rice.