Slow turning over 6.5 GM Diesel

I don't want to beat a dead horse about connections, but...one of my work trucks is a 1987 6.2 diesel. Last fall into winter I had a hell of a time with slow cranking. I thought I checked all pertinent connections, but still had to jump it at times. I thought that since it wasn't used too often we compromised the batteries that were only 2 years old. I replaced the batteries but that did not fix the problem. Prior to this problem I also converted the truck to top post batteries from side post batteries. However, when checking connections I neglected to check the connection between the cable and the top post terminals which is tightened with 2 bolts per terminal. Those 2 bolts were very loose and slightly corroded on all terminals. After I cleaned and tightened those connections The truck started like new.

Not saying this is your problem, but I want to make sure you exhaust all cheap solutions before you replace the starter.