I'll try this here because all the truck forums give me, "You got me?".  Anyway my 02 F-150 4.6L/5speed 4x4, was bought new and has been a good truck.  A couple years ago I noticed on a long drive at interstate speeds that when I downshifted or upshifted into 3rd that it would refuse to go in gear without letting RPM's come down.  It has progessed to the point that yesterday was hot and grinded every time I hit 3rd.  Pull over let it cool off no problem.  Cold outside no problem.  Drive for an extended period at speed (20+ minutes) or hot outside - grinds.  No other gears do it, and as a Mustang owner too, I know 3rd is the one you tear up in those too - usually a bent shift fork.  But once bent it is always bent, not like this which is obviously heat related.

Anyone seen this before?  3rd gear shift fork or syncho?

Also I have changed the fluid twice (syn) with no avail.  Some told me heavier oil - no sure about that one.  Right now its IV Mercon ATF is memory serves.