My 2005 F-250 6.0 PS transmission is shifting very hard. When I start from a standstill it feels like the transmission does not want to shift. It will finally shift but with high rpm. After it warms up it shifts a little better but not like it should. Also I've noticed that even driving down the highway the rpm's are higher than normal for my truck. I just replaced the external filter and put in 8 quarts of Ford tranny fluid but have not noticed any difference. There is 128,000 miles on my truck. I bought the truck used about 5 years ago and have had no tranny issues until now. Any ideas??