Possible causes include:
-Improper fluid level
-Manual linkage misadjusted or damaged
-Low main control pressure (internal leakage) This could be the forward clutch center support seal rings failing,
-Forward clutch assembly burnt, damaged, leaking
-Valve body bolts loose, too tight
-Valve body valves sticking, dirty
-Pump damaged, leaking
Low pressure is caused most likely by an internal leak, or a valve body problem which could include either a gasket leak in the VB or a valve issue.
It is pretty common for people to have to rebuild their VB's
How many miles do you have on your transmission. Has it ever been rebuilt?
I would first make sure your manual linkage is properly adjusted.
Check your control pressure to see if it is within range, if it isn't the best you can hope for is a valve body problem because they are usually easy to fix.
http://www.explorerforum.com/forums/showthread.php?t=117998
If all that is OK, I'd either rebuild the VB or replace it with a rebuilt unit.
If that doesn't help, you most likely have an internal problem that can't be repaired with the transmission in the vehicle.
