I thought I had fixed my P0171 when I changed the vacuum switch in the dash. But it turns out that this was at the same time as we got loads of snow and I have had it in 4wd since. No that the snow is gone and I switch it over to 2wd, the P0171 is back!?

I guess this means that one of the vacuum lines to the trans or 4wd acu is gone bad, but who is it? And why only in 2wd?

Hoping for some help