'05 Sienna Shift Gate not holding position

While recently driving, I down shifted from "D" to "4", but shifter jammed trying to go back in to "D". Once vehicle was stopped, I managed to force the shifter all the way up to "P", but something may have broken, as there is no lock of shifter in position. (If in "P", it will move freely from sided to side; same for "4" to "D", which will cause downshifting if not aware of shifter position.) Cable does not appear broken, and dashboard gear indication follows shifter position. What may have broken, and is it something I can replace myself?