My trans on my 06 dually hit 300 just running down the road, running 75-80 mph, no load on, stopped and let it cool down but now the fluid smells burnt, I'm concerned that I smoked the trans seems to still shift's ok but get's hot fast now, looking for any help just hoping didn't cock the trany.

First check your fluid level. Warm trans to 160F to 200F. Engine running in park, trans fluid level should be near the bottom of the |XXX|HOT|XXX| band at 160F and not over it at 200F.

If the level checks out OK, pull the trans "To cooler" line out at the radiator tank and look inside the radiator cooler for debris that could be plugging it. Most likely it would look like a wad of cardboard paper. Pick it out and reconnect the line.

What I was needing to know is if I need to rebuild or replace the trans after getting this hot? what kind of damage may have been done?