Transmission or Transfercase?

[Lightning]

I have a 1976 Chevy blazer that i use for a plow truck and somthing locked up and i cant get it to move now.
I was plowin snow on saturday, and it was working fine, but i parked the truck and went in the house for awhile, when i went back out to finish up, i started the truck.. but it acted like it was under a load... just sitting there in park...
it was odd but not enough for me to care... so i went to put the truck back into high range.. and the lever would not move.. i put the truck into drive.. and it was acting like the parking brake was on... would not move at all.
so i shut it off, and then i was able to go from high range to low range in the transfer case.. and if i put the transfer case in neutral the truck will roll... but the motor is acting like its fighting something????

so what could have happened? i cant figure out why it would work fine.. then when i shut it off.. .and come back to it later and it flat out wont move....