Neutral safety switch will have nothing to do with that at all. Primary function is so that your starter is disabled whenever your in gear. This makes so can only start in park or neutral. I assume you have automatic transmission?
it was giving no problem at all and all of a sudden it just wont go into reverse. every other gear works great. the fluid is clean and smells right. is there something else i need to look for or do you think its internal. it is automatic
Look under truck down where shifter comes thru. There should be a shifter "link" that connects it to side of transmission. Maybe linkage is bent in a way that doesn't allow shifter to move in direction of reverse on transmission. Maybe have someone shift with key in on position and see if you can see how shifter moves. Possibly ran over something that bent that link ? If not may have problems with reverse in transmission. Keep your fingers crossed!