Wont go into 1st gear while in drive!

[USNstrokin31]

New to the site, been racking my brain for a while now on the possible problems to fix the issue, but just can't figure it out. I have a 2000 f-350 and just put a new suncoast transmission in a few months back. Everything worked fine until recently when i noticed from time to time my truck was a little rough to take off from a stop (like it wasnt fully engaged into first) and then started doing it more and more. I got rid of my transcommander and went with a TW 6-position, and have an edge evolution CTS programmer that i put back to stock. The truck wont take off when I have it in drive/OD, but when i bring it all the way down to 1st manually it goes just fine, and I can shift to second, then drive/OD and everything works just fine. Any help or suggestions would be much appreciated!