I had an older GMC Van and discovered (Through a transmission shop) that a nut inside the transmission became loose. This prevented the shift linkage from working properly. The column shift would rotate like a clock dial and engaging any selection was hit and miss. Luckily the nut didn't come off and ruin the transmission. In that case the oil had to be drained and the pan removed inorder to see inside. This would be the opportunity to change the filter inside as well.
* I can only guess at what the problem could be. Changing the oil could be futile if work is required later but changing the oil and replacing (or cleaning) the filter may resolve the issue.