Speaking form a bit of xp on this one...sadly. You can easily change your fluids and the filters. They are a snap but a bit messy.
When I was having this issue I took it to a dealership adn they gave me the DRB codes because I asked for them and luckily I did because they wanted me to swap out the whole tranny when all is needed was a new valve body. Basically it went from 5500 repair to a 1100 repair....which I could have done myself but did not feel comfortable doing. I would take it in though...the diagnositc is only around $90 here. Should be around that by you....should be. As many others have said in previous posts to have a 45RFE go out is rare. Not saying it does not happen but it is a tough tranny, if it wasn't they would not put it in the Dakota's and hook it up to a V8.
As for the shift kit...I didn't put one in mine but I kind of wish I did. If you have the spare coin, might as well, it won't hurt anything and could help you out down the road. The only shift kit I would recommend is the one that Yellow Coyote and TJKJ put in...but the name of it is escaping me at this moment....Transgo perhaps?
Keep us posted.