Most likely neither. My truck had this problem(mostly after starting from a dead stop) I'd get like three clunks till i started moving.
Repair: slipjoint in driveaxle needs greased. not to indorse this site but here is a link http://www.explorerforum.com/forums/...d.php?t=106307
your drive axle probably isn't a two piece like that. I left mine attached and unclamped the boot,then i squirted a ton of moly grease in the boot. held boot in place with zip ties and jumped up and down on rear bumper to work the grease into the joint. No more clunk!!!
Hope this helps, it takes about 10 minutes and will rule the slip yoke out if anything.