This question comes up more often than the tire size question.
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 weenie 2wd Hardbodies.
HF37 - H190 axle found in weenie 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.
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 gear
H190 - 190mm ring gear
C200 / R200 - 200mm ring gear
H233B - 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.