Igotaproblem

Hey need some help. We just finished the big job of replacing the oil pump and water pump on a 1999 Chevy Silverado 2500 LS 6.0 L. We had to drop the differential and remove the pan in order to replace the oil pump. Well, all is put back together and in place now. The truck starts and drives fine, but now the 4wd won't engage on truck. Given weather conditions currently, we kinda need it. Looking for reasons why it wouldn't work, as we didn't disconnect anything from the differential, merely undid the 4 bolts holding it up to remove the pan. Any advice and places to start would be appreciated. Thx.