A speed sensor issue would cause the computer to put the transmission in limp mode. From what you are describing, you are not in limp mode. More than likely you have an internal transmission problem. The first thing to do is check the fluid level and condition. Make sure the transmission is at operating temperature and the van is on a level surface with the engine running when checking the fluid level. Next thing, you need for a transmission shop or a shop with a high end scanner to scan the TCM (Transmission computer) the transmission computer will display very important information about the transmission such as the CVI (Clutch Volume Index) values. This is very helpful in determining things like broken snap rings in clutch drums and worn friction elements and slipping clutches. A diagnostic needs to be done on this trans. How often has the transmission been serviced?? Also, if you need to top off the fluid, use ONLY ATF+4 fluid.... NOTHING ELSE!!!!!!
Post back with the results of the scan and about the fluid level and condition.