What kind of grease have you used for the spline shaft for the driveshaft? I know that a lot of the early F150's had a problem with clunking and the fix was to drop the driveshaft and grease the output shaft splines.
Rich

 

anything would work for a while. Im not sure what they recommend, but if you want something that will hang in ther for a long time, i would use grey/ black grease with molibdinum (moly) in it. Thats the stuff the grease bull dozer blade joints with... and they take a pounding and grease stays in place.

 

..Steve...Have you ever had any clunking with your supercrew? I've just hit 119k and u joints are tight, it just sounds like the splines are "sticking"
Rich

 

here's a TSB covering the lubricant used.

05-26-10***CLUNK OR BUMP WHEN STARTING FROM A STOP