downshifting issues! Help!

[Jackedup67]

So I've been driving the 2000 crew cab 5.7l for a few weeks now back and fourth to work and its been great. I went to get in it this morning and after about 20 mins. of driving, it started having some issues, like it doesnt want to go into od and when it finally does on like the highway, Ill put the go pedal to the floor and it just slowly accelerates in od, like it wont kickdown at all. If anyone has experienced this or has any idea what this could be please chime in. I really need to get this figured out cuz i'd hate for it to leave me stranded or get worse. Thanks Guys!