It sounds to me that it is in too far on the top & putting pressure on the gasket, thus binding.
Loosen the 3 bolts on the top hinge & push the door out some & tighten...check were your at & repeat.
The problem with discussing this over the internet is that something obvious to someone, may not be to someone else.
I to, like to remove the striker while aligning the door, hood, trunk, ect. It is a part of the latching system, not a means of guiding the part into some other place...If only every bodyman would relate, agree & abide by this simple rule
One word of caution is: On these trucks, the nutplate behind the post doesn't have a cage around it so when you loosen it, it will rotate & will fall down once the striker is removed.
A door with a new gasket will be hard to close for awhile, you may need to leave the striker out an 1/8" or so until the gasket "takes shape" & then move it in flush with the cab corner. Hope this helps, Lorne