Well not to help make you wanna give up on it but my truck is now acting similar to yours. On the dyno tuning and driving on the road, I get up to that 3500 and it starts missing and backfiring. Don't get me wrong the sync still should be checked/set but it may not be the issue. I discovered I had the wrong spark plugs so I replaced them with correct ones. Haven't been back to the dyno yet, just driving I haven't seen much change though. Does appear I can power through the missing and it cleans back up at 4k

 

That sucks but im not giving up that easy, other than clear coat issues theres not a spot of rust on this truck and the frame is surprisingly clean as well for a truck with 260,000 miles on it. So far my truck is running good and my a/f gauge shows a good mixture through all the rpms. You probably have but did you check all the sensors and make sure they were clean and working? Also if you have a way to check the air/fuel ratio that would help determine if the backfire is caused by rich or lean conditions to dial in whats wrong.
I just got a new wiper motor put on so i can start driving it again, ill probably report back here anything that comes up.

 

The dyno has an A/F gauge at the tailpipe, and the tuner is using the installed 02 sensor, which was put in new, and they're giving conflicting readings, one rich, one lean, the truck one being the rich one, they installed a wide band direct to the tuner laptop and bypassed the in truck one and confirmed the truck one is reading correctly, at this point we're unsure

Sorry I must have missed a post, you got your issue fixed?

 

I think it was post 8 that I made that kinda explains my fix. All i did was relocate the intake temp sensor and cleaned all my throttle body sensors and i havent had the truck try leaning out and backfireing since ive done that.

 

I guess I did read that, for some reason I didn't fully process that it had resolved your issue, but good deal either way, I'm running all stock sensor locations. I debating just going through and replacing alot of them, truck is creeping on 30 years old, sensors of unknown age, truck sat for 3 years. The distributor pickup is first one though, since the cap and rotor were all green and nasty the pickup coil could be crappy too, and I already have a new one hiding somewhere in my organized mess...... I need to get my scan gauge hooked up in this truck and see what it can read, might be able to find some false sensor readings on a cold engine

 

[QUOTE=dodgetruck2;3547304]I guess I did read that, for some reason I didn't fully process that it had resolved your issue, but good deal either way, I'm running all stock sensor locations. I debating just going through and replacing alot of them, truck is creeping on 30 years old, sensors of unknown age, truck sat for 3 years. The distributor pickup is first one though, since the cap and rotor were all green and nasty the pickup coil could be crappy too, and I already have a new one hiding somewhere in my organized mess...... I need to get my scan gauge hooked up in this truck and see what it can read, might be able to find some false sensor readings on a cold engine[/QUOTE

I believe you can figure it out and good luck.