1946 Sunday Times-Signal (Zanesville,
Ohio) 12 May I. 7/1 [They] told
citizens here that somebody was ‘out
of pocket’ in Bowie and Miller
counties the nights of the killings,
and urged them to recall whether
anyone they knew was missing on those
dates. 1973 J. PETERSON Sicilian
Slaughter 53 Her hands shook as she
dialed. But her connection was out of
pocket. 1974 Anderson (S. Carolina)
Independent 20 Apr. 1A/1 If you..have
ever been sick and the only doctor is
out of pocket for the weekend, then
you know we need more doctors. 2002 A.
PHILLIPS Prague III. viii. 229
Five-day weekend for me, Charlie,
starting in eighteen minutes. I'll be
out of pocket until Tuesday.
I first heard this in the US Southern states, and some attest to it being common on the East Coast. This is a regionalism, and while quite old, it should be considered informal and would not be universally understood by all US speakers. Those on the West Coast, for example, would be largely unfamiliar with it, and only use "out of pocket" to refer to self-expense payments.
To "pull one's leg", as a saying, does seem to have the etymology you describe; every source I can find states that it dates back to the mid-1800s in England, and refers to physically tripping up another person, which puts him off balance, possibly makes him collide with others in awkward ways, and generally makes him look foolish. It quickly evolved to mean achieving that result - making a person look foolish - regardless of the specific means used. The most popular means to do so is to tell a deliberate plausible non-truth which, if believed, would lead the person react foolishly.
"Pulling one's plonker" by contrast seems to be a much newer term, still considered slang and rare in American usage (it's most common in British and Australian vernacular). It's one of many examples of introducing a sexual connotation to otherwise "innocent" idioms and sayings.