i had the same thing happen to me, i was turing on a hill nad all ofa suddon my car just had no go. it started coasting back even though i was in gear. i had it towed to my house and the speedo would go up to like 20 mph and the car would start and rev fine, it just wouldnt move(even in gear). i took it to the shop and they fixed it in like a few hours. turnes out it was a gear on my axle. i had worn all of the teeth on the gear down and it was just spinning and not giving power to the wheels. i hope this helps.
P.S: it only cost me $167 i was excited, going into the shop thinking tranny trouble.