I think I changed the shift points around, just when it shifted. No pressures.
You need to install a tow/haul shifter and that would help a lot. You can just manually shift in the meantime so it's not as strained. Consult the manual for your tow ratings and probably go about 750lbs lighter as far as your max if you're not using tow mode.
Now, I had tow/haul in the wrong pin and pulled my Subaru home with it. 6klbs with 4.11s, and it did it no issue with tow haul. So, hold your gears one way or another until you get regeared, then use as needed. Watch trans temp until then especially.
Once you get your wideband in you can drive it up my way and we can actually VE/MAF tune it for the day. Then we can mess with power enrichment. I think I turned the threshold down for PE for your truck to give you some protection. I'll update this post once I double-check on that.