I want to cry. I just paid off my little Ford Ranger XL and now it won’t go in reverse. For about a year, when I place it into a gear (it’s automatic), it doesn’t sit right on the appropriate letter, rather, it goes between the two; i.e. to go into drive, it sits between D and N. Anyway, tonight, my husband placed it into reverse and heard a loud noise like the tailgate falling. Once he got out and saw that was not the case, he tried to place it into reverse again and it would not work. It goes into all the other gears, but not R. Any thoughts? Oh, by the way, I have no money. 
Sounds like the linkage isn’t working correctly, which could simply be due to damaged and/or misaligned linkage or it could be that a transmission mount broke and the transmission itself fell, which would explain the thunk. This could be a very simple repair if you’re lucky.
You don’t state the year of this Ranger. If you have a three speed automatic in this truck, you have probably broken the low/reverse band. One diagnostic is to put the transmission into manual ‘low’; accelerate up to 10 or 15 mph; and let off on the gas. If the truck freewheels, i.e. engine speed drops immediately, the absence of the low/reverse band is indicated.
If you can get good lock in ‘park’ and it is working in ‘drive’ you probably do not have a shifter linkage problem. A transmission technician could probably do a diagnostic quickly and give you the news. It is possible that something broke in the low/reverse band servo which could be repaired without removing the transmission. But if the low/reverse band has broken, the transmission will haveto be removed and disassembled to replace the band. You might as well have a rebuild done at this time.
Come back to us with additional information and we can provide more ‘replies’.
Follow up: Even if you have the AOD overdrive transmission, the same problem can occur. It is not a band in this case but the reverse clutch pack. If the clutch pack retaining snap ring lets go, the transmission will not have reverse only. In the case of the AOD, the reverse clutch is not used as a backup for first so the freewheeling previously mentioned will not occur. However, there is nothing that could be done about this without removing and disassembling the transmission.