98 Ford Explorer won't move in reverse

Sometimes it may just be the fluid level, if you have drive , drive it around the block a few times, then park it on level ground and hold the brake and go through all the gears slowley, two times. put it in park, open the hood and check the fluid level, the dip stick in the back by the fire wall, take it out and clean it, read the stick. SOME need to be checked in Nutral, if this is the case do so but be carefull that the truck has stopped moving when put in nutral!!! if it is low only put in the right fluid, and a little at a time, while it is still running DO NOT OVER FILL !!!!!!!! Also sometimes it is a SENSOR problem, and in most cases not to hard or costly to replace.