Rear end has a loud clunk

Check your universal joints for binding especially if the truck has been lifted. Then check the backlash in the rear diff. Raise both rear tires off ground with truck on stands rotate tire until driveshaft begins to move if more than 1/3 turn have rear diff. Checked internally.

One of the mechanics at the dealer where I bought the truck found a TSB that seems to have worked to get rid of the clunk. I found some info on another site and am including it here.
Here is the TSB---real easy repair--you can do this yourself---works for a while. Basically the slip yoke is sticking and then pops loose and you hear/feel a clunk.

FORD: 1998-2004 F-150

ISSUE:
Some 1998-2004 F-150 4X4 vehicles with a one piece drive shaft may exhibit a clunking or grunting noise and/or a shudder sensation from the driveline. The condition will typically occur during light to moderate acceleration from a stop and/or braking. This may be caused by the slip yoke not sliding smoothly on the transfer case output shaft splines.

ACTION:
To service, lubricate the slip yoke with Teflon Grease (D2AZ-19590-A). Refer to the following Service Procedure.

SERVICE PROCEDURE
Road test the vehicle to verify the condition.
Remove the driveshaft per Section 205-01 of the Workshop Manual.
NOTE: INDEX (MARK) THE DRIVESHAFT YOKE TO AXLE FLANGE FOR REASSEMBLY.

Lubricate the slip yoke with 7-13 grams (0.25-0.46 oz.) Of Teflon Grease (D2AZ-19590-A).
Re-index and reinstall the driveshaft per Workshop Manual.
Check the transfer case fluid level and add MERCON Automatic Transmission Fluid XT-2-QDX if necessary.
Road test to verify the condition is corrected.

SPICE