2002 B3000 Transmission trouble

Just noticed some harsh upshifting from the auto trans (as though the vacuum modulator is bad), then the "O/D off" light starts blinking...owner's manual says have it looked at or risk damage. I don't have a service manual yet, please tell me this is something easy to fix in my driveway. Dealer is 35 miles away, not sure I'd trust them anyway; 2WD, 44k miles, never abused...Any ideas?

More info - the fluid looks/smells like new...