All 6 misfire

Misfires can be caused by a problem with the ignition or fuel system and sometimes can be hard to find. Since all 6 cylinders are misfiring I believe it is something they have in common and share. Some of the common causes you have already addressed like the cap and rotor. I'am assumming one of the things the cylinders share is the ignition coil. Before changing it test it and make sure the resistance is in specs. If it is good then look else where like the fuel system, a vacuum leak, or lack of compression. To begin trouble shooting the fuel system check the fuel pump. You can do this with a fuel pressure tester and the pressure should be within specs. If not the filter could be clogged or the fuel line kinked or the pump could be bad. To check for vacuum use a vacuum gauge. The needle on the gauge should be steady and read 17-22 HG. You don't mention how the truck runs, but if the motor runs good and steady it is probably not a vacuum leak. Finally you can do a compression check on the cylinders. You did mention the truck has 171, 00 + miles so this is a possibility. Remember don't replace anything unless it test bad. Hope this helps :)