Yes! what he said. The transmission is stuck in two gears, your going to have to remove the top covet of the transmission and figure out which two it is and manually pop them put. I will wager the shifter is worn, when you remove the transmission cover look to see if the shift gates are all lined up or if one is offset to the others.It sounds like two gears are engaged at the same time, locking up the tranny. The wheels going in opposite directions is the differential not turning the input shaft.
He "said" lots of things but some don't match-up so my money is also on stuck between gears or in 2 at once.He stated earlier that he could shift in and out of all gears. Also stated the shift cover was off and nothing wrong was noted. Stuck in two gears is along shot there.