OK, this is information we compiled over on our 4x4 site. Since it seems there are a lot of axle related questions here, I thought I'd post up for everyone. Mods, if you want to sticky this, feel free.
Information compiled by Desert Rat:
What Axle Do I Have?
The axle code is located on the door jamb sticker on Nissans. If it's not there, there are visual ways of determining what you have. On some older trucks it's on a plate in the engine bay above the fender well on the passenger side.
HF35 - H190 axle found in 2wd Hardbodies.HF37 - H190 axle found in 2wd Frontiers
CA41 - Found on 720's - This is a C200/R180 combo with 4.11 gearing. Some 720's have the smaller H190 rear end that's identified by not having a removable rear diff cover. R180 front ends in 720's are different than the Hardbody and Frontier versions, as the diff is nearly centered. 720's use unequal length halfshafts and are not interchangeable. All 1986.5+ halfshafts are interchangeable left to right.
CA43 - found on 720's and Hardbodies. C200 rear / R180 front, 4.375:1 gearing
CA46 - found on Hardbodies and 4 banger Frontiers - C200 rear / R180 front, 4.625:1 gears
CA45 - found in 2003+ Frontier 4x4's. This is a C200/R180 combination with a new gear ratio - 4.5something:1 The C200 was upgraded to 31 splines on this model from the previous 29 spline unit.
HG43 - Found on Pathfinders, V6 Hardbodies, and a few '99 Frontiers - H233B rear / R200 front, 4.375:1 gearing. On the 2000+ 33 spline axles, the HG43 has a 4.363 ratio, however this is not a common axle code on the later models.
HG46 - found on auto equipped pathfinders, 96-97 hardody 4 bangers, many Xterras and Frontiers. This one has some variation. Pre-2000 versions are the H233B axle with 4.625:1 gearing (31 spline axles). 2000+ is the H233B axle with 4.636:1 gearing (33 spline axles). Typically comes with an R200 front axle, with the exception being in the '96-97 Hardbody pickups - those had an H233B / R180 combination. Axle shafts in the H233B were 31 spline up to about 2000, where they switched to 33 spline shafts. All Xterras and R50 Pathfinders received the 33 spline axles. In Frontiers, it can be a crapshoot if you have a '99-'00 model, though it is believed that the switchover was at or near 2/00 production date.
HG49 - found in mid year 2001+ Frontiers and Xterras. H233B / R200 combo with 4.90:1 gears.
I've learned recently from posts in NICOClub that there were some Hardbody 2wd's that appear to carry the C200 axle, but I've never actually seen one. If you have one, it's a rare bird. I'd love for anyone to post up their information and describe the year and model (I4, V6, XE, SE, etc.) of the truck that carries the C200 rear in the back of a 2wd so I can update my FAQ on my website.
ARB lockers are available for the older 30 spline C200's and all H233b's. They're also avaialble for the R200 front, however no traction device is out there for the R180.
Calmini LSD's will work in an R200 front. (A Calmini LSD is a slightly modified '87-89 Turbo 300ZX LSD)
Lockrights will only work in the older 31 spline H233B.
Detroit used to make a locker for the older 31 spline H233B, but those are getting scarce.
Up to 4.875:1 gearing is available for C200's and R180's. Up to 5.143 gearing can be had for H233B's and R200's. AC and SLR are the two companies offering gears, or you may be able to get them through Nismo.
Let's say you don't have the doorjamb sticker.
H190 - small 3rd member type axle found in 2wd's only (excluding Desert Runners, Xterras, pathfinders)C200 - removable rear diff cover, 8 bolts. Found in all Nissans (yes, even a few early 2wd Pathfinders)H233B - large 3rd member type axle found in most V6 and a few 4 banger 4x4's, Desert Runners, and 2wd Pathy's and 2wd Xterra V6's.
The H233B 3rd member changed from 9 bolts to 11 studs to hold it to the housing beginning with the 1990 models. Somewhere along this time (maybe around 6/89, the bolts holding the ring gear to the carrier changed from 10mm to 12mm, making a difference in replacement gear sets.
What do the numbers mean?
The number in the axle model indicates the size of the ring gear in mm.
R180 - 180mm ring gearH190 - 190mm ring gearC200 / R200 - 200mm ring gearH233B - 233mm ring gear (yes, that's about 9.25"!)
How do I figure out my gear ratio?
Put a jack under the diff and get both wheels under the ground. Draw a chalk line on the tires where they would contact the ground. Rotate BOTH tires by hand at the same speed one exact revolution while counting revolutions of he driveshaft. Just under 4 1/2 turns will indicate the 4.375:1 gears. Just a bit over 4 1/2 turns would indicate 4.625:1 or 4.636:1 gearing. If you have a late model, just a tick under 5 turns of the DS would indicate 4.90 gears.
LSD or open?
If you lost the sticker, get that jack back under the diff. Turn a wheel with the truck in neutral. If the other wheel spins in the opposite direction, you have an open diff. If it spins in the same direction, you likely have some sort of traction device - probably an LSD.
Contribued by OnlyOneDR on N4W:
Tractions aids can be found for the R180 in the 720, Nismo, Quaife, Kaaz, and Cusco sell LSDs, mostly because the R180 was under the earlier Zs and newer Subarus. Not sure if you can swap the bolt retainer in from an open R180A carrier to get it to bolt into an HB or Frontier axle. I did that trick to get my 300ZX lsd into my R200A.
The HB and Frontier front axles are "A" axles, (e.g. R200A, R180A). The A designates an offset carrier. The R200/R180 are different because it is centered and does not have axle tubes on either side.
Contributed by OffroadX of N4W, AC, and XOC notoriety:
I'm not sure that the A in R200A etc. indicates an offset diff, from what I gather, it indicates a short-pinion diff. The Patrol uses offset front (and even rear) H233 diffs, but they're not designated H233A. I have seen in the Nismo catalog that the R180A was developed with a shorter pinion (by 120mm) to reduce driveline angles vs. the regular R180.
The R180A axle designation is RBThe R200A axle designation is RC
C200 and R200A gears ARE INTERCHANGEABLE. Yes, the same part numbers are used for the same ratios in the Nismo catalog for both diffs.
Best I can tell, 10mm ring gear bolts were phased out of the H233B diffs 6/86.
There are 5.571 and 5.875 gears available from Nissan/Nismo for the H233B
Regarding the H233B, the ONLY difference in Nismo 5.14 gears are at the 1990 model year. There were originally only gears for pre-90 diffs. AC sold a modified version of those for post-90 diffs, but a couple of years ago Nissan finally started making 5.14 gears for the 90+ diffs.
4.90 Xterra gears will go into any 90+ diff too