Chain - As said in the email, I've not had this problem. But the only place the ATF could be getting into the speedometer cable is through the speedometer gear. However, I didn't realize that your truck is a 4x4, and the speedometer gear doesn't plug into the transmission on a 4x4. Instead it plugs into the transfer case.
The pic on the left is what the speedo gear looks like, and the pic on the right is where it goes in the t-case. But, you should check your t-case as it may have more ATF in it than it should, which could cause this problem.
Gary, AKA "Gary fellow":
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Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI