My does not want to engage in drive (D)

My does not want to engage in drive (D). Sometimes the gears will catch in D, and it will go, but other times it just revs and will not catch. It is an automatic transmission and drives decently in R, first, second, and then, when I am already driving in second and shift to D, it drives, but it has trouble taking off from D. I have replaced the 46re transmission. I have replaced the shift linkage, the torque converter, and several other parts. I cannot identify what is wrong. Furthermore, when it is in D and R, it rolls pretty much freely, like it is neutral.