1998 Dodge Ram wont shift out of 1st gear

Transmission has been absolutely flawless after being rebuilt over a year ago. Went to get inspection sticker and did not have reverse lights. Mechanic says I need back-up light switch and or nuetral safety switch located on the tranny. Drove my truck to the dealer to get the part no problem, they put the new part in and I had reverse lights. Got my sticker and drove off. Could tell the trans was shifting different and had to tac to 3000 to shift into 3rd. To back to mechanic and he flipped how dare I accuse him of my tranny problem, all he did was replace the switch. Here I am a week later went to leave for work and now it wont shift out of 1st no matter what. Replaced the output speed sensor, still same problem. I need help please