will a 2nd gen. rear axle fit my 3rd gen truck? I know one has drum and the other has disc brakes but will they physically fit? They would be both 1500's

 

I'm not sure if the ABS connector will fit, and if you don't have that hooked up the truck throws a fit. Grab a tape measure and get the WMS (wheel mounting surface) to WMS width. Same as the distance between the centre of the leaf springs.
I looked into it as well but ended up buying a rear diff from an 07 with 30,000 km's for $350 or $400.