[dsraven]

is your truck 12volt? is the solenoid/bendix 12 volt? is the end of the starter gear ground up? possibly the starter gear is slipping or the solenoid isn't strong enough to "pull it in" all the way. yu could try bench testing to see if the drive engages all the way.
check all the connections from the battery to the starter, I mean like take them off, clean then and re-attach. ensure there is a good ground strap from the battery to the engine and frame. a lot of starter issues are a simple poor connection or a battery post terminal that "looks good" but when taken off it is black with corrosion so there is a lot of resistance. also. a lot of guys will overlook the ground cabling because they feel it is the positive terminals that do all the work because usually they are the corroded parts. in reality the negative cables do just as much work so everything needs to be bright and shiny. a star washer under the frameand engine ground cables is a great idea as well. it helps keep the connection tight and making good connections. lots of guys will forget to install a good ground cable to the frame and the engine as well.
just some thoughts