Well, he finally agreed with the diagnosis posted by a couple of you guys here. He replaced the new "used" shifter with a brand new shifter. I made sure to tell him I told him to do that 2 weeks ago and could have saved us both a lot of aggravation
I have not driven the truck that much (yet), but it appears to be driving fine so far and it looks like the problem has
finally been solved. Thanks to everyone for your input.
Justin