7.5 GM gear Swap

I have a 7.5 GM rear axle in my bucket T. The gears are 2.73 with 32 inch tires. I need to do a gear swap to 3.70s. I cannot find any letters stamped on the axle tube, probably due to brackets being cut off and new ones welded on. I did find BP 05 01 W stamped on the top of the passenger side axle tube. I am guessing this relates to a date maybe May 2001. Anyway I was going to buy a whole axle with the 3.70 gears and just swap the carrier and the gears. The salvage yard said he had to have the 3 letter code from my axle to know what axle to sell me for the swap. I am not a chevy guy and cannot understand the reason for this. What am I missing here. I know I can buy a ring and pinion and spacer or a Wolverine gear that is thicker that will bolt onto the carrier I have. I want to buy the whole axle and be able to use the 3.70 carrier. I might luck up on some larger brakes as a side.

 

try early 80's S10 w/4 cylinder 4/5 speed - they usually had 3.73's

 

Or 4.11's...

 

I am not having trouble locating an axle with the 373 gears, just don't know why he needs to know what axle I have????