I've had this 96 cherokee 4.0 SE for about a year now and I've been trying for a couple months to figure out why it not only backfires out of the exaughst but also the intake, the only code it gives me is p0171 and I have changed everything related to that code but I can not for the life of me figure this out, I get terrible gas mileage, struggle tk get to 50 mph without back firing, mis firing and feeling like I'm gonna drop the tyranny or something, any help is appreciated

 

I'd start with timing.

 

o2 sensor?

 

Check compression. Then vacuum for clogged exhaust.

 

Two things I can think of:
Plugged cat - Easy to check, just unbolt the exhaust before the cat.
Timing issue.