After raising my xj three inches I have to adjust the transmission. instead of using a sye kit, I would like to lengthen the driveshaft and / or put a double cardan.
Good idea?

Lower transmission.
driveshaft will be 400-500.

sye is best but also requires a driveshaft with it.

Hack n tap for a street rig. Full sye for offroad

You can use a hack n tap that uses a front drive shaft that you can get junk yard for less then $50.00 . they good for trail riding and some medium off road stuff .

400-500$?
I have a dear friend with a workshop that does these jobs. so I'd pay for it with a beer (or 2, or 3 or more....).
Would stretching the original shaft only a good job?

If you have the means you can do anything.

I fabbed the front shaft on my cummins i set up a dial indicator when welding it get it true just to be sure i had drivline shoo check it for balance..

Im not sure lengthing the shaft will gain much. if its to short then youl correct that issue but its still going from point a to point b.
Since you can make your shaft i recommend investing your money in a real sye and putting your time and skill into fabbing a shaft. Reducing the length from ppint a to point b will help your drive line angle.

Also KISS
Keep it super simple, crank n bake baby. A welder built rig will always be better.

https://www.jbconversions.com/products/sye/np231j_short_sye.php

Lengthening the stock shaft alone for a 3" lift is likely not enough to remove vibration issues. They do sell (or you can make) and double cardan jointed shaft with the stock Slip Yoke at the transmission (https://oliversdriveshaft.com/shop/r...j-wj-cherokee/). I don't think it's the best solution, as slip yoke eliminators are cheap in the long run. The JB conversions SYE is a tick about the rest in quality.

Lots of options for you....just choose what's best for your budget and abilities with your friend's shop.

So if he goes with a SYE from JB Conversions, how would he know what part number to order?

https://www.jbconversions.com/produc...1j_std_sye.php

How do you determine which Part number is for a specific jeep? (this is also for my knowledge bank)

XJ's generally came with an NP242 or NP231 transfer case. NP242 has the full time option; NP231 is part time only. JB conversions does not make an NP242 Slip Yoke Eliminator, so you'd have to go to another vendor for that one.

Here is a little information:
http://www.novak-adapt.com/knowledge...er-cases/np231
http://www.novak-adapt.com/knowledge...r-cases/np242/

No need for the Super Short SYE on the XJ....so if you have an NP231, you want Part #16-1200-1942O (which is what I have) which gives a yoke for a double cardan style shaft with 1310 u-joints. I guarantee you do not need 1350 u-joints...those are for massive tired vehicles.

And...if you're going for a new driveshaft...I've learned to just go to a reputable local shaft shop. I have been burned by a few online companies touting top quality parts, but using cheaper components which had slop as new. I was happy to spend an extra hundred locally to get all us-made NEAPCO and Spicer parts. Since you're local...Bill's Englewood Driveshaft did me right.

Oh yeah I knew about the 2 different x-fer cases, but there is 4 part numbers there... so the difference is the u-joints that will go with it... You're right, i wont need 1350 u joints.
one last question for us, what driveshaft will be used once that particular SYE is installed?

Sorry to hi-jack for a minute...

my xj mounts an np242, I remind you that I live in Italy and it is not easy and not even cheap to have everything you have in the USA. in addition I would not like to open my np242 to make the change. if I spend little to do a good job, I'm happy, but if I need to, I also spend a little more. I thought that by lengthening and possibly adding (but not necessarily) a double cardan I would have done a good thing. I would like to completely replace the cylindrical part of the driveshaft with a longer one. i apologize again for my english (thanks also to google translator, but just a little bit, i want to improve my english)

Lots of options for you....just choose what's best for your budget and abilities with your friend's shop.[/QUOTE]

it's a very specialized workshop in high level precision mechanics (I don't know how to say in English, but I think you understand).

I shared the jb super short link because I'm looking into a black box or 231/231 doubler.
there are tons off options. Sometimes phone calls must be made to truly understand what's needed. To much internet clatter to make decisions on how i spend my own money.
Make phone calls order parts and burn 🔥 what's needed.
Stepping out of the clatter and into the fab world is where its at

As far as what drive shaft. Idk a conglomerate of shafts i have from various pickups anchored to what ever yoke bolts on the sye with what ever tube fits that size yoke

looking online I read that "hack and tap" is fine from 4.5 inches up ... I only have 3 inches!

Hack and tap has Good solid following. For good reason jeep folks are tight asses. Theres was a time when I thought i might run a hack n tap but I didn't really understand the reason why I mighg run it.
like you i fall into the less than 3" lift category where im not needing to correct driveshaft angles. I also have a shackle relocation and lobger shackld this to helped my driveshaft angle.

I hVent seen much on the hack and tap failing but I do know now that I wouldn't run one if I did need to its just to
It may help some with driveshaft angles but its still a hack and a weak looking on.

We are blessed here the jeep and offroad world is booming technology is advancing aswell as garage fabrication. Things i do in my own garage I didnt even think possible 10 years ago. Ive completely built my jeep from steering to suspension with very few bought parts. I love it but sometimes hate it as it sit on jack stands for a month straight

I'll do some digging and see whats going in in Brazil. I know the Icelanders are building baddass formula hill climb rigs with what appears to be limited what they have for parts and stuff but still they stomp the crap out of stuff built over here.

Can you post pictures of the front and rear drive shafts so we can see the angle of the u joints ?