The purpose of this study is to determine whether nutritional (cholecalciferol) or active vitamin D (calcitriol) supplementation improves vascular endothelial function in patients with stage IIIB and IV chronic kidney disease with vitamin D insufficiency or deficiency. The investigators hypothesize that the use of calcitriol supplementation will result in improved vascular endothelial function as compared to cholecalciferol supplementation.