If you have an 18 spline output shaft and t-case they have a high failure rate and are known to break. The 25 spline assemblies can handle a lot more power and last longer.
We can convert your 18 spline trans and t-case to take the 25 spline assembly either in a rebuild or if you want to send us the parts. If you want to do it yourself, and send the pieces, details of the parts we need are below for us to machine for the conversion. Note, this is not an easy thing to do. The trans needs to be opened for the swap and many parts need to be removed and reinstalled. A press will be needed for the t-case coupler as well.
In the pics below we have an 18 spline shaft on the left and 25 spline on the right.
Pics of parts we need for the 18-25 spline conversion: