I have just finished getting a replacement transmission in this morning. Same problem - no reverse however all forwards were good.
I looked at just getting the reverse piston repaired however after removing my transmission oil pan I saw that there was a fair amount of ground up fine material in the pan and filter.
Didn't screw around. Changed out the transmission - took 4 hours. Found a great condition alternative transmission with 75K miles on it.

Here is a foto of the filter. You can see the residue on it. If your filter looks like this - don't hesitate - get another transmission or a rebuild.