Im rebuilding my posi unit, I bought a used unit as I have an open unit in my truck now. The shaft that holds the two smaller gears in place has a hole for a set screw, but I do not have a set screw in the unit. When I open up my rear will there be the set screw in the open unit or do I have to find one?

 

are you talking about the pinion shaft? in an open diff, the spider gears slide over it and it hold everything together. about 1" dia and 5" long? yes, yours has a set screw installed.
here's some pics. https://dodgeforum.com/m_697667/mpage_3/key_/tm.htm