Fill the case to the top with Diesel fuel and let it sit for about a week. Bolt a piece of 1/4" X 2" steel flat bar about 3 feet long onto the 4-bolt output flange with an equal amount of the bar sticking out on both sides. Use a big dead blow mallet to thump the bar back and forth in both directions until the output shaft frees up, then put the transmission in 4th. gear so the countershaft will turn and free up the bearings that support it. Once the transmission turns freely, dismantle it completely and replace all the bearings.

I'll be surprised if the gears don't whine so loudly when they run that you'll need earplugs to drive the truck.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!