re: Truck Rear Differential question

first off your information presented is lacking to say the least.

is your truck 4wd
have you check your ujoints on the rear driveshaft for slack and/or wear
have you checks your rear axle bearings for wear, dont know much about Fords but if its a full float axle you would have 2 bearings per side that need to be maintained and preloaded properly
ford tcases are known to have output bearing problems have you checked that.