Tax, you don't know what tax is,come on down to Ontario and we will charge you another 8% on top of the 5 your already paying, all in the name of political stupidy!
The receivers, I have no idea unless you can find a part number stamped in them somewhere.
Axels it would be best to go by bearing sizes, but lugs can give a indication. 4 lugs are usually 2000 lb, and would have 7" brake drums if fitted with brakes, but have a option for 5 studs on the same axel
3500 are usually 5 lug but have a option for 6, but use a 10" brake drum
5200 and 6000 lb have 12" brakes and are usually 6 lug, but can be 8 lug also or house trailer style.
7000 lb axels are 8 lugs but still use 12" brakes.
This is generally, but they may be exceptions.
You can compare the parts here
http://www.rigidhitch.com/ to get a feel for what goes with which size. If there is somthing specific that you want a new one of I may be able help you find one.
Ken