Just so I understand you correctly, when Marlin does the HD upgrade, your saying he uses a bearing that are the same inner and outer diameter as the stock bearings, but is upgraded (has more ball bearings) correct?

This is correct, however I dont know about the bearing count, possibly its better heat treated metal or the race is wider, I dont know for sure, but yes same id and od, just stronger.

I want to make sure, because I will probably be ordering one of these kits from Marlin if the tranny innards are in fact 20 years old, and in need of replacing.

This is a good idea, but I am not sure if Marlin offers any kits featuring the HD parts, I think the HD option is only available as a full rebuild. You would need to call in and ask to find out for sure.

Also just as a side note, what is the deal with these tranny's that have the aluminum center section? My friends and I have pulled several W56's out of 85 vehicles, and as far as we can tell, this is the first one we've ever seen. Where these only produced in limited numbers? Where there several factories making these trannys, but one decided to go with aluminum as apposed to cast? What's the deal?

Well to my understanding ALL 11/84 - 11/85 EFI 4WD 5-speed trucks came with W56's featuring the aluminum center section. You might want to check with the transmissions that you removed to verify that they were not the original tranny by the transmission code stamped on the tranny. The first two numbers are the month, next the year, and then the serial number of the trans. For example, 06502456 would be a June of 85 and the 2,456th trans built in that month. I'm willing to bet that the cast iron W56 trans you removed from your 85 vehicles were not OEM, but replaced from some 86-95 model truck.

My only guess as to why Toyota used the aluminum is that it was the first debut year for the W56 and Toyota wanted it nice and flashy. Maybe the first EFI prototype show trucks used aluminum in their first prototype W56 trans..

BigMike