mine has been giving me some similar issues and until getting the new case in I've found a temp fix. Take out the t-case lever and then you look into the hole and look at the forks. They need to be in a certain spot to let either one shift. You pull back, push over and then push forward. Sometimes it blocks up. I pull the lever and drop it into the other slot and push it where it needs to be.
After getting it locked a few times I realized that I had 512's in the front and 462's in the rear. I had been looking for the 462's for a while and they were in the zuke already. I don't drive on the road much so I never noticed. At slow speed it doesn't show at all but it loads the drive train and then you can't just push the lever.
I actually didn't clip the lever back in and just left it sitting.
Oh..the fix is normally to replace the plastic shifter sheet.