If the speedo works, that tells me the transmission is working. That leaves the differential which is part of the trans housing. The most common problem is the spider gear pin comes loose. As a result, even though the trans is tring to move the veicle, the differential doesn't move the vehicle. The bad part is this. To check the problem, the differential cover (behind the trans)needs removed. You can remove it to inspect, but in most cases, there isn't enough room to replace parts. If that is the case, the trans needs removed. However, that is where I feel the problem is.