Our 1999 Pontiac Montana minivan has been surging slightly at a steady speed. No check engine light was on. When we got it home yesterday and tried to leave a few hours later, the shifting lever would move, but the van will not go into any gear. The motor makes no different noise when we move the lever up or down like it is even attempting to shift into a gear. It has never clunked during shifting and has always accelerated well when given gas. We're wondering about linkage? Or more seriously, transmission?
If it happened all at once, chances are it is a sensor or control module issue. If the check engine light is on, have the computer scanned to determine if there is a problem with the TCM. Also, make sure the trans fluid is clean and full. As you mentioned, it would be a good idea to check the linkage to make sure it is connected and properly adjusted.