Clutch pedal won't depress??

Okay here's my issue, I got a S10 truck from a
friend about 3 weeks ago. It is my understanding
that the clutch was replaced just before I got it
from him. I drove around for a couple weeks
almost in "safety mode" because it's an older truck
and I didn't know how well it ran.

After just over a thousand miles of driving I
decided it was solid, started right up, ran
well/smooth, shifted nice, ect. Feeling good about
it, I decided it was good to go for a little longer
distance drive.(hour away) I was on the highway
for about 45 minutes at 65-75mph, 5th gear entire
ride. As I approached the exit I attempted to
downshift into 4th after coasting down a bit and
the clutch pedal was stiff as could be, I couldn't
push it in even standing on it pretty strong. (I am
6'5" 410lbs) I popped it out of gear and coasted
down the ramp, stopping short of the end, and shut
it down. First thought was to bleed the slave
cylinder and see if I could get pedal to depress. I
opened the bleeder wide and not a drop came out.
Pedal still hard as a rock even with bleeder wide
open. I noticed that the drain plug had fresh
seepage and that it was not even hand tight. As
well as a few various other bolts in the same
vicinity.
So it definitely ran dry and seized up, god only
knows how long. Obviously something seized
inside the clutch. What I'm wondering is how do I
diagnose what's actually broken instead of just
throwing a whole new clutch in this thing when I
might just need a throw out bearing or a pressure
plate?