I talked to the guy who owns Brutal OffRoad in Bandera (a big mopar guy that not only knows everything mopar, but especially the trucks)...

he gave me the detailed instructions on how to measure for proper distance and and said it'd be somewhere between 2-3".

I looked at the wear pattern on the slip joint, and I can see where the the seal originally rode, then it changed as the stock springs wore out; and by taking 2.5" off the shaft, it'll put the slip joint right back to where it was originally.