Like the title says, my 91 Ranger with 2.3L and 4 speed automatic transmission wont stay/go into overdrive. It will shift into 4th @ approx 55mph but will almost immedeatly shift back to third. Any suggestions/ideas? Im usually good with these things but Im lost on fords. Could it be a shift modulator or something vacuum controlled? I bought this thing to get decent gas mileage and Im probably not getting that rolling around @ 3000 Rpms!

 

Need help, fuel mileage plummeting