DC recommends transmission fluid with the "C" additive to prevent shudder. Without knowing what the rebuilder did, it is difficult to say whether or not that is what you need.
If it were mine, I would drain the transmission completely and put in the recommended product. It takes about 17 quarts but might help and it would give you a chance to flush out any debris that shouldn't be there.
There are several brands that make Chrysler approved products.
Mopar of course, Valvoline and RedLine all have fluids with the Chrysler specification.
I wouldn't be unhappy with the 45 mph shift, mine shifts whenever it feels like it. Some times at 30 and the next time at 55.
I hate it when it goes into OD at 30 or 35 then immediately locks up the TC.