Budget and location. I could point you at a half dozen local shops here in AZ that can do the job.
Keep in mind that paying a shop to do the work you will be looking at a 5-figure job if you want it done right. Tires and wheels, a lift for the rear to match the front, sourcing a front axle, springs and shocks, building the suspension. There will be a lot of time to figure the suspension as there are no kits that make this a drop in. Don't forget the front axle. Transmission will probably need a rebuild to attach the transfer case, oh yea a transfer case. Driveshafts. Steering linkage, hopefully reuse the existing steering box. and a mountain of little things from longer brake lines, longer speedometer cable, the transfer case shifter.
It's one thing to do the work yourself with a donor 4WD giving up the parts you need. It's another to pay a shop to source the parts and pay the labor.