Wont go in reverse
 

Hi, im new to the forum, my dads having a problem with his 91 f-250 LD. Im guessing it has the mazda transmission and not the zf5 since its the light duty 250. I havnt gotten under and looked. I may do it in a little while but guess it doesnt matter. Anyways reverse isnt working. It'll go into gear on the shifter but it doesnt do anything. I would like some ideas about what someone with experience on transmissions would think is wrong, and if it would be something i could fix on my own if i pulled the transmission. Thank you all in advance.