mr. white

I've got an 05 with the 4.2L automatic transmission pushin 141k. Today on my way to work I noticed my transmission wasn't wanting to bite into 1st gear until I hit about 2000 rpm. When it finally catches it jerks pretty good then after that it's all good. I took it for a spin on the highway real quick and every other gear works fine. It's just starting out from a stop its what's givin me problems. When I purchased the truck used from the dealer it had 86k on it and shortly after owning it I changed out all fluids, so I've put about 60k on that new tranny fluid and its never acted like this before. Any ideas or recommendations?