grinding noise when downshifting

Assuming you're talking about a MANUAL transmission...

I must admit that I've not worked on repairing/rebuilding a transmission since I worked on my 4 speed transmission in my 1965 El Camino.

However from the days when I rebuilt the transmission of the 1928 Model A that I restored from body off in the early 1950s the usual reason for downshifting clashing was operator error. In the days before the synchros in transmissions if you didn't double clutch and match the engine speed with the road speed the transmission wouldn't shift down without SERIOUS grinding of gears. I got so I could shift up and down without using the clutch by sensing the correct engine speed for different shifts but it was easier to merely double clutch.

SO.... I would wonder if the synchros or the modern day equivalent of them are worn in your transmission ....??? The serious difference in rotational speed between the shafts of the tranmission mean that the gears will clash since the synchros aren't synchronizing the speeds of the gears so they can mesh without clashing. That also means you coul be filling your transmission with metal shavings or even pieces of the teeth of the gears if you continue to let it clash.

Any transmissi> I have a 2000 V-6 4x4xe kc that I just started to notice a grinding

You have a synchro that is starting to wear out. While I don't believe you have much to worry about because it will only get worse gradually and won't strand you, I would suggest you quickly get the fluid changed. Use something like Redline Synthetic MT-90

as a replacement. Also, I believe the factory calls for a fluid change every 30,000 miles; if that schedule was not followed, it is usually a synchro or two that start to wear out. If you want it repaired, you're probably looking at close to a $1,000 bill. If it really bothers you, consider finding a used one at a reclamation yard.

Wrong fluid?