In my 99 auto trans Ranger, I have be in neutral (or park), foot on brake, and completly stopped. Even slightly rolling will prevent it from going into low range. When I first got the truck, the trans position sensor was off a bit- noticeable by the backup lights not coming on unless I held up on the shift lever a bit. There is a special tool for this, but I simply did this- BLOCK THE WHEELS, PUT PARKING BRAKE ON, put key on (engine OFF!), put the trans in reverse, then crawled underneath. Loosen the two small bolts that hold the sensor. Rotate it back and forth to find out where the backup lights go on/off. Use a mirror so you can watch. Put the sensor in the middle of that range, and snug the bolts. Worked for me, have had no problems since then.