OD Off wont work, slow to shift into OD

[RocKrawler]

My OD Off button on the end of the shifter will not work, light will not come on. No CEL lights on the dash. And it is now starting to take longer to shift into OD on the highway sometimes. Its a 1997 E350 with a 7.3L and to the best of my knowledge everything is stock (coming from the 50+ yr old lady I bought it from, who got it from a leasing company that used it as a commuter van to & from DC. Any plugs on the trans I should look for & if so where? Fuse or relay? Button on column typically go out? HELP!