There is a plastic piece between the shifter rails on the t-case (Dana). It sounds like that broke, not allowing the two to move as one. That's what mine did. I twin sticked it while I had the opportunity.
To check: Just take the cover off from the inner floor board and shift the t-case, the base of the shifter should go to a plastic piece linking the two shift rails, so that when you shift the t-case, both rails should move together. This was the part that broke on mine.