I've recently purchased a 1986 Cherokee (winter beater) that was a 5 speed manual but has been swapped with a 4 speed manual by the previous owner (by mistake, junk yard error). The yard gave him the corresponding rear axle from that transmission but I've read elsewhere that the axles "must match" somehow or there will be problems. Can I put this axle under my Jeep without detriment to the front axle or drive train? Will the tranfer case grenade if I use the 4 wheel drive as it is? This 4x4 game is all Greek to me so any advise would be helpful.

yes if the gear ratio in the front and rear axle's are different you can tear the whole drive train up. do you know what the gear ratio is in the axle you want to put under it and the gear ratio of the axles under there now?

It is only common sense that if the front axle is trying to go a certain speed and the rear axle is trying to go a different speed then something has got to give unless you are driving on a very slick surface.
My first guess would be the transfer case would grenade. It may be a ring and pinion or even a driveshaft but for sure something is going to break.
The current ratio is easy to check and the imported axle can be confirmed by the seller.
"Everything must match." Any questions please post them up.

Not only do your gear ratios have to match so do your tire sizes!!!!

Good point sk8.

Thank you guys! Is there a mark on the axle housing to indicate the ratio by code? I haven't found a site yet that defines these explicitly, just what years have what ratios and styles (Dana 30, 35, etc.) on the front and rear but don't define any indicators.
thanks Sk8, good point, but they're all 265/75-15.

The rear diff usually has a metal tag with the gear ratio on it. They often get disgarded when the cover is popped off for a gear oil change so if it is not there I would not be too surprised.

If the ID tag is missing you can still easily get the ratio. Since this Jeep is new to you,you should be doing a gear lube fluid change anyway.

Pull the differential covers and rotate the ring gear, you'll see a number like 41-9 etc.. thats the tooth count. Divide the smaller into the larger for your ratio.

This is true with any Dana gearset, some aftermarket companies dont mark them.

Very cool, thanks for the info Overland XJ!

Even if the gears are not marked you can count the pinion gear and the ring gears yourself and divide them.
I am not sure about the Jeeps as my meager knowledge is 67-72 Ford trucks but on the 76 F150 I had and the 86 Bronco the rear gear was different from the front gear due to them being from different makers. Ford 9" rear might be a 3.50 front Dana 44? might be a 3.55 or something like that. It is real close just not exact. Are the Jeep Cherokees like that with say a dana 35 rear end with 3.55's and a front end from Chrysler? a 3.54? I don't know the Jeep gear sets so I am guessing at the ratios but do you get what I am asking?
clint

if they are close, like 3.55 and 3.50 or something like that you should be fine. if you are running 3.55 on one end and 4.88 on the other, thats bad juju.

With reasonably close ratios, like 3.5 and 3.25 you can compensate with different sized tires, like 31s and 29s, I've worked on a F-250 with that setup.

Also, a some of the non-Dana companies mark them like 3|55, or just 355.

On Fords a 3L55 would mean it is an Equl-lock or limited slip with a clutch type set up
clint

Yes, not all, but that's not an L, it is a large vertical line. I've been playing around with a chrysler mid-size axle, and they write them that way sometimes. Also, the old AMC 20s do sometimes, I think. Generally, though, it's pretty clear what part is the ratio.