if you can't do a leakdown test, try putting a little oil in the cylinder (via the spark plug hole) before doing the compression test. if that brings the reading up in line with the others, the low compression is most likely due to sticking or worn rings. (like darin said, though, that doesn't mean the headgasket isn't leaking to the outside.)

if the reading is unchanged, it could be the headgasket, but could also be a valve not seating properly.

1994 4x4 22RE W56 truck