Ford: 99 ranger-automatic/no reverse, d1 d2, pressure tester

Question
I have a 99 ranger xlt 4x4 w/118,000 miles.It runs fine and goes forward in 1,2,3&od.The 4 wheel drive also works fine/I have no REVERSE! I have spoken to a couple of mechs. about this. The one i trust the most said there is a piston or servo in the bottom that probably blew the seals or may have cracked. Any bright ideas,I'm tired of parking where I don't have to back up.

Answer
Steve that is rt they have a reverse sevro but the best  way to test this is to rent a trans. pressure tester and see what kind of presure you have  it should be 90 psi in reverce.35 to 45 in park and netural 60 to 70 in d/d1/d2 and the sevro can be change from the outside.