96 c1500 305 Bogged down in drive with no overdrive

I think you may have a couple different problems compounding. I think the power problem might be with your engine and not the trans. Say for example plug converter. The trans not being in the right gear could something like maybe , just maybe the transmission is the wrong years to your truck or the linkage is wrong or if it's a rebuild them the rebuilder assembled some wrong with valve body. Something like what you have going on would be best looked at by a good shop with experience for mechanics not somewhere like Firestone or Goodyear. Not to say those places don't. But for the most part they have younger guys doing front end, tires, and brakes