A "clunk" like hesitation when accelerating from a dead stop

Ford F250 SD V-8 5.4 Triton.  When accelerating from a dead stop, there's a slight hesitation before the truck bites and goes forward.  doesn't happen at any other time and vehicle runs smooth afterwards.  No other problems with shifting, etc.  I can't remember the name of the part, but it runs around $400+ and costs about the same to install (supposedly a pan to install).   there is a place in WA state that sells a rebuilt (can't remember them either).  Does this ring a bell?  It's one of those.. if I heard it, I'd know it sort of things.