Hello. This is my first post to the forum.

After replacing the fuel pump (needed it), and a tune up (cause the damn thing wouldn't run after I fixed the fuel pump), my truck now runs sluggish and idles pretty low. Like 200rpm in R or OD. I recently drove out of town and noticed that the transmission doesn't want to down-shift out of a higher gear in order to accelerate. This is the only noticeable symptom I have found. What could be my problem?

Also, my radiator was replaced during the tune-up because it was leaking. My fluid levels don't appear to be low, but @ 130k miles, I'm sure that the truck is due for a transmission service.

Any suggestions on how to diagnose??