Ford recommends an updated Teflon grease on the blue spline teeth. I had Ford do mine, was only $100ish out the door. I had Ford do it, mostly to replace the metal band holding the boot with an equivalent as opposed to a cable tie.
Hey, mine had this issue and now has the same problem again, its a pain in the *** getting yoke to slide back on the splines once greased.. I beat the hell out of myself getting it back in. My first time it solved the clunk which at the time the dealer had now idea what the prob was, it lasted around 25,000 miles, now its back and is bugging the hell out of me. Time to do it again.
drill and tap a grease fitting in there.. The yoke will slide in like a glove(relieves it from getting air bound) I grease my spline every 15k miles because im anal and hate the "thud".. Grease fitting is the way to go.. Drill and tap the driveshaft end, easy peasy takes 2 mins to grease yoke now)
1/4"-28 standard grease fitting. I believe its a 1/8" pipe thread/tap... I drilled and tap on the driveshaft side with the male end(spline side).. As far as grease goes.. To be honest with ya this last time around greasing it I just used regular temp texaco motor/gear grease i had laying on the shelf.. Dont think it really matters, that spline isnt getting that hot to take away the properties of the grease- its there just to soften the clunk, you are always going to have a tiny bit of play, maybe not enough to feel it while taking off from a stop but some. Welcome!
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