FORD has a TSB on this...not sure of the number, but this fix includes a new separator plate with bonded gaskets and a new blow-off valve. About 30$ a piece from Ford. It addresses no 1st or 2nd gear, slow into reverse, etc.
The 2 piece factory gasket on the valve body blows out in different places causing a loss of line pressure in the valve body.
Don't know what year Ranger you have, but this is a COMMON...I repeat, CCCCOOOOMMMMOOONNNN problem with the Ford 5R55E transmission and 4.0. Go to a transmission shop and they'll tell you that you need a rebuild. 90% chance it's just the valve body. Another problem that causes your symptoms on this tranny are loose VB bolts. That's it. 90 inch pounds. They have a tendency to loosen. EVERY time you do a fluid change on this transmission, check the torque on the VB bolts.
Get the Superior or Transgo shift kits to really make this tranny great.
Better yet, purchase a rebuilt VB from centralvalvebodies.com which includes all of the Sonnax brand (tranny company) upgrades for 229.99 if you tell them that you're a member of Explorerforum.com.
There is a WHOLE write-up with extensive pictures/instructions at Explorerforum.com. Another GREAT Ranger/Explorer forum.
TRS is my favorite...I have over 1,000 posts for the past, oh, 8 years...but I leave for a while and forget my old username/password and have to start all over again!! Really are good guys here that will do their best to help a brother out.