Can you change the bellhousing on 2003 gmc truck 4l60e transmission to a 1998 4l60e transmission from a Chevy astro van?

The bell housing should be the same. You're probably thinking of the other end of the transmission... there is a difference between the 4WD and 2WD transmission, where the 2WD transmission has an output which goes directly to the driveshaft, while the 4WD transmission has a mounting face for a transfer case, and the output goes into the transfer case... just find a 4L60E from a 2WD vehicle, and spare yourself a lot of hassle.