Toyota Repair: 1996 4Runner, rear axle seal, high mileage

Question
My 1996 4Runner (SR5, 229,000 miles) runs very well, but has a sort of ticking or thumping noise (not quite sure if it's a tick, thump, knock or rattle).  It seems to come and go, being more noticeable when I'm going at a slower speed; a high mileage oil seems to help keep it quiet, but it generally comes back. It appears when I'm accelerating or have my foot on the gas. It isn't there when I'm coasting or idling. It seems to get worse when I've been driving at a higher speed for awhile (such as on the highway).  I can feel the thump/whatever underneath the front passenger side floor.  The car has had regular maintenance, most recently for its 221K check (that included replacing a rear axle seal and a rear brake job).  I replaced the timing belt at about 210K, but the noise predated that replacement.  Any ideas?

Answer
It's kind of hard to diagnose noises unless there is very specific information, since you don't know which it is how do you expect me to help without even hearing the noise, so I'll have to take my best guess and hope it helps, it sounds like it may be an internal engine noise, possibly a connecting rod noise or piston noise, it has a lot of miles on it so this could be the case. Especially since putting in different oil seems to help for some time and also the fact that the noise seems to be louder when accelerating which puts more load on all the internal engine parts, sorry, can't be of more help with the information I have.