At about 30 mph the truck downshifts into low gear when slowing down. I changed the idle air control valve about 2 months ago and it ran fine. Now it wants to downshift a little too soon when stopping at a light or stop sign etc., (most of the time at around 30 mph.) Any suggestions or ideas?? Could the new idle air control valve be messing up? Or could it be the TPS or something else??

I'm afraid I'm going to mess up the tranny if I don't fix the problem. It's not a jarring stop, but you can feel it kick in a little too soon.
Thanks

 

Use an Ohmmeter to test the TPS from it's center output to each end over full rotation looking for intermittant operation.
Besides this sensor, the PCM has few other ways to tell what the trans should be commanded to do along with it's speed sensor, to make the decisions when to shift.
Of course there is more to this in program control but it dosen't mess up like that unless someone changed the shift skeds with a programmer.
Good luck.