Hmmm, then I'd go ahead and try the "STOP LEAK" stuff; rather than have a shop tear your truck apart and still might not find it the problem (and make you pay for the "labor" anyway). The STOP LEAK stuff might not be the "correct" fix, but it may be an "alternative" fix.

I guess you've got three(3) options:

A. Do nothing and live with it (just monitor coolent level).

B. Try the "STOP LEAK" stuff, and see what happens.

C. Have an automotive shop do "exploritory surgery" on it, and have your checkbook/credit-card ready regardless of outcome.