my 99 winstar had a tranny replaced 25k ago by dealer. now tranny seems to slip going into 2 gear. when leaving 1 gear it just revs and wont go into 2. the thing is it doesnt do this all the time. when i bring it to mechanic it runs fine of course. it seams to do this when the fuel gets real low. i know it might sound stupid but can the fuel level or pump have an effect on the slippage, or is it just going to get gradually worse

I cannot see how fuel level, or the fuel pump can have anything to do with the transmission slipping.
The ONLY connection that I can possibly find is if you have a low voltage issue in the electrical power system......starting with the battery and battery terminal connections......then on to the alternator.