1986 d21 rough run issue

[Lamina1982]

1986.5 d21 2wd with 4cyl 24zi.
Truck had been running fine then the was sputtering alittle while driving, pushed gas hard few tikes it back fired and blew mufgler open. Made it hoke and now it will start and idle all day no issues but as soon asyou put any load on it trying to drive/move engine sputters and wants to die.
It even sat for a montha nd started up oerfect and will stay rinning no issues at all. Just cant seem to drive as then its sputtering and wants to die.
Hooked up a fuel.pressure gauge today and stayed a steady 40.
I thought maybe injectors so got a rebuild kit and didnt make a difference.
Cant quite seem to figure it out.
Thanks

[Desert Rat]

I'd be hunting for massive vacuum leaks first.