Truck is dying when shifting into reverse and throwing oxygen sensor code 44

[MMA205lnl]

Ho hoping to get help. My truck started backfiring. Then I put fresh gas and it got better. Started backfiring again, would get better if I shut it off and started driving agsin. Now it's throwing code 44 and dies when I shift it into reverse. Sometimes it idols fine and sometimes not. The truck is a 1985 with a 95 5.7 v8, has a 86 ot 87 computer, 4 speed granny tranny. It's a fuel injected throttle body. I have cleaned the egr valve, checked for vacuum leaks, changed fuel filters. Any ideas? Motor was brand new 15 years ago and only has 20,000 miles on it now.

 

[Charlie]

 

[1987 GMC Jimmy]

44 is lean exhaust. Sounds like it could be sucking in some extra air via a vacuum leak. What all have you checked, and how did you go about it?

 

[4WDKC]

any exhaust leaks?