2003 GMC Sierra Gear shifting problem

I recently changed out the transmission myself. Everything working fine, except one thing. When shifting gears the display in the truck would not show up properly, I would be in drive, yet R for reverse would be chosen, and my reverse lights would be on in the back of the truck, id have to shift to reverse, then slowly to neutral, and slowly again to drive, then it would take a few seconds, and drive would show up properly.

Let me make sure you understand that the proper gears were being selected in the transmission, it just wasn't displaying properly in the truck, causing electrical issues with making the truck think it was in reverse for example when it was in drive, i.E reverse lights on in drive, and not being able to select 4wd low due to the truck thinking it wasn't in neutral.

I changed the neutral safety switch, thinking it was that, on the side of the transmission. It is now a little better, the line under the chose gear on the dash shows up, except now, if I want it to show im in say Neutral, I have to chose neutral, then go down one gear to drive, then back up to neutral and it will be in neutral on the dash. This is the case for all gears, and I cant seem to get into 1st gear by selecting it all the way down at the bottom of the selections because I can't go down a gear to do it, its as if there's something in the cable, maybe my cable is stretched? I've tried adjusting it tighter, and looser at the cable beside the frame of the truck where the connector is, but I just cant seem to get it. What I dont understand is why I didnt have this problem until I changed the transmission.