94 ford f150 trans

I bought a 1994 ford f150 3 months ago has about 100,000 miles. If it sits
a few days at acceleration it begins to viberate like you are running on the
left side of the interstate warning lane. It only does this when you accelerate
and after 3 or 4 miles it goes away..I don't know anything about fords, so
if someone could give me a clue as what is causing this I would appreciate
it..Trans is automatic with overdrive and acts the same out of overdrive.
thanks r dobbs

If it were mine I would service the trans and put synthetic fluid in it. That seems to fix a lot of the weird stuff like that..