besides wheel base and being 4x4 and 2x4 is there a diff. in the frames. i have a 98 1500 chevy xcab 4x4 and wanna change it to hd 2500. and what all is involved in doing so?  

There might be an extra crossmember in a 4x4 to support the t-case. Also 2wd have coil spring suspensions. 4x4 have torsion bars so you might have to modify the front suspension on the 2500 HD frame.  

The frame is different on HD model than it is on 1500's.  The front control arms are wider and mounted in a slightly different location on the frame on hd2500 and 3500's.  The front differential mounts are different also because the hd models have a 9.25" ring gear instead of 8.5" up front.