Just went in for a rebiuld and ended up with a new trans. Apparently mine was either towed and extended distance or more likely had gear oil put in it by the previous owner. these trans should go the distance if you can avoid these two sure killers. My new one shifts like a dream but is no bracket racer. The synchros (backing plates) need a moment to line things up and then are good to go into the next gear, forcing the shift is a no no.
My trans guy said he has never had problems getting parts or an entire rebuilt trans. He had mine the next day after finding mine impossible to dissasemble....probably from gear oil.
The earlier ones without the larger input shaft and clutch didn't accomodate the rear most bearing with an oil passage. The newer ones do and you don't really need the extra qt of fluid. Most rebuilds from that era have the passage drilled into the housing. I am not sure but I think the 2000's and up were the improved version.
The final drive ratio is the same as the five speeds BTW. The sixes have an extra gear in between 2nd and 4th that makes towing better when upshifting. Some think it is a higher gear at the top but such is not the case.
Its a bulletproof trans if you don't kill it with standard gear oil or towing w/o disconnecting the driveline.

Kurt

 

what does towing without disconnecting the drive line do to a standard transmission?

 

If it is not built for being towed, you will destroy the tranny, as it is not getting lubrication. It does under power, but not being towed. Some manuals can be towed 4 down, but not these trucks.

 

i still can't see how towing a standard trans vehicle will hurt it, an auto yes, but gears are turning in a standard which will whip up the lube

 

I remember reading (owners manual I think) to tow, place transfer case in neutral.

 

I know with a lot of manual transmissions if you tow the vehicle, only the main shaft in the upper rear part of the transmission is spinning.

The countershaft and input shaft aren't turning because the engine isn't running, and the countershaft gearset are the gears that are below the oil level in the case...

So basically no oil gets to the main shaft/output bearings, because the only parts moving are all above the oil level.

I'm no expert, but that's my understanding.

 

if the main shaft is turning so is the counter shaft. the 5600 is pretty tough but if you abuse it it will break the common places ive seen them break is either the input shaft or the main shaft will snap. also if you use the wrong fluid or shift it without the clutch alot you will kill the syncros.