A tranny shop will just want to pull and swap or rebuild. Try changing the fluid, and add an inline filter to catch all the junk that will flush out. I made one from a remote oil filter kit with a standard Ford oil filter, and it worked great. As an added bonus, it did increase the capacity by about a quart as well.
Also, add a cooler, or if there is one already, make sure it's in good shape and is mounted where it gets enough airflow.
Check out the transmission forum for advice also.