i have a strange problem with my transmission - first here is the
overview of my pickup:

1992 toyota pickup
22R engine
4-cyl. 2.4 liter
2 wheel drive
5-speed manual transmission
150,000 miles

a few weeks ago, i discovered that it would occasionally pop out of
first, but that if i physically held the stick in place it wouldn't. 
then about a week ago it started popping out of 5th - but again, this
was rare and i could hold it in place.  now a few days ago, i have
additional problems: i find that i can only partially engage 3rd and
4th - the stick itself just barely moves into position - as if
something is blocking it.  even when the engine is off, i cannot get
it fully into 3rd or 4th (but 1, 2 and 5 are fine).

i am not sure when i changed the manual transmission fluid.  i got a
new clutch at about 120,000.

my questions are:

what is the problem, do i need a new transmission, or new syncros, or
what?  whatever the problem is, how much should i expect to pay for
the repair.  i see that lots of parts (ie. transmissions) are
available on ebay - is this something that someone relatively
mechanically inclined could change?

You could remove the shift control cover and inspect the shifting
forks to see if they are worn, broken, or bent.  You could buy new
forks if that is the case.  If it's not that, your bank account is
probably going to take some punishment.  You could pick up a complete
used transmission, preferably one from a truck with lower mileage.