Well, this is a first for me although I'm by no means an auto trans expert.
My fiance normally drives this car, I drive it once a week, maybe. When I drove it on Sunday, all was well. But, as I posted earlier the transmission started acting up on Tuesday.
Well, I noticed, after letting it sit overnight, that the battery was very weak (original 6 year old battery) when I tried to start it on Wednesday morning. This was also the time the problem showed (it did not show tuesday night). So, my philosphy being to always fix what I know is wrong first, I put a new battery in yesterday. I waited about 5 hours before restarting and all was well. Changed the fluid last night as it is almost due anyway. And, went to bed hoping for the best.
Got up this morning, the trans problem is gone. Since 1st worked properly when that gear was selected, my only thought is that the low battery was somehow causing some voltage spikes/dips that was freaking the tranny electronic control unit out and not fully engaging first when Drive was selected and the computer was to control it.
Oh well, I've staved off that replacement for a bit.
As to your question about the lean condition, I run an oldschool maxxpower kit with uprated injectors and an S-AFC. I don't even know if those guys are still around???
And, as to why i don't have an upgraded trans in this, I have a levelten trans in my yukon. I hate it. The thing shifts too hard on the 1-2 shift. It's fine for that truck and for towing but not for everyday driving. Everytime someone rides in the truck, they ask me what's wrong. I'm not sure if the 4runner trans is the same way or not. I'm not going to do that to the 4runner until I can ride in someone elses. Many people have had problems with level10 in the past. It took about 20 phone calls and 3 months for them to credit my core charge back to my card. So....unless you just do the valve body (which isn't going to prevent all trans failures) you're going to pay about the same in the end. My fiance drives the car very gently, so I figure I'll just wait....