Engine codes

[bluzchk66]

Hi there...new to this forum but hoping someone can help. I have a 2004 Liberty with 162K on it....auto mechanic pulled codes when the engine light came on for a second time (first time, the "other mechanic" said it needed new ****** fluid...flushed the system etc.) and then the light came back on again a week later with codes P0700 and P0740. Next mechanic pulled the same codes plus P0706 and I pulled P0839 this morning. He is saying it needs a new transmission but a lot of info I have been reading says it could be sensors. It's not slipping, just seems a little sluggish when going up hills. The tach doesn't "race" so I am second guessing the diagnosis. I am a single mom of a college sophomore and really don't have 3,000 to put in to it at the moment. Thoughts, advice, suggestions welcome. Thanks!
Jennifer