I did this conversion on my 08 2500 w/auto 2 months ago. After all the comments about just getting another truck...this was BS..the conversion is straight forward.
Mine is stock height.
You will need to pull the tailshaft housing and install a 4wd housing and the shorter output shaft...this is done with trans in truck.. you will need to lengthen the speed sensor appx 12". That is all that is needed trans wise. I used the manual t-case, got the shifter from GM and the floor pan has the area for the shifter already indented..easy install...just pulled the seats, console and carpet back then cut out with a cut-off wheel. As far as front frame area....you will need to cut the forward hole in the frame on the passenger side..the rear hole is already there. Make a sleeve or cut a piece of pipe to weld into the hole that you put in the frame and install the bushings. The front diff bracket on the pass side is easiest to install with bolts instead of the studs that are installed...the bolts fit in from the outside then into the frame and bracket. On the drivers side you will need to weld in the upper bracket by the gear box..got this from a wrecked truck and also the rear lower sub brace at the control arm...side to side. The rear driveshaft will need to be shortened or get 1 from a 4wd, the slip yoke is different too. The front axles will slip right in your hubs.
I spent appx $3500 total to convert my truck to 4wd...it all looks stock and works great. If anyone is interested I will get some pics of all that I had done and send out.