Ahh Crappy, you know how I feel about them Rangers.
She's a beauty, aint she? Nuthin' quite like wringing out that little Pinto engine and squealin' down the road. I don't mean squealin' the tires, I'm talkin 'bout squealin' like a piggy as you tear up that quarter mile in just under five minutes.
"C'mon Bessy, skuh-weeeeel!"
I still drive my Ranger. An '86 with 359k miles on her. Yeah, it's slow and looks like someone opened a big 'ol can of fugly all over it, but that baby's an heirloom in the making.
I know you said you replaced the air filter. Did you happen to notice if there was any kind of obstruction in the air intake tract? If there is a rag or dead rodent (you said it was in a field) stuck in the ducting, that will cause a definate power loss at anything past half throttle.