I believe my DANA 70 housing is damaged and while searching CL for a replacement I found a DANA 80 SRW from an 02. What are the downsides to swapping out? Weight and rotating mass, anything else? Should it bolt up?

It should be a straight swap, the pinion might be longer so that might affect driveshaft length. 

Might be change in brakes. 1989 to 2001.5 is all drum brakes. After 2001.5 and up is disc brakes.

Upper and Lower U-bolt plates are different (due to larger diameter of axle).

Is yours currently disc or drum rear brakes?

 

Currently discs, as would be the donor axle.  The listing says the axle is from a 2500, was that an available upgrade?  I read that it was, but it wasn't what I'd call an irrefutable source of information, either.  I think my dad talked me out of buying it since I don't actually KNOW that I need it.  Of course I would think it would be just as easy for me to resell it later if I don't need it.  If I didn't have so much on my plate for the next two months I'd drive the 4 hours round trip to go get it.

Automatics got D70 (10.5" ring gear with 3.5" tube diameter)

Manuals got D80 (11.25" ring gear with 4" tube diameter) [Actually a Hybrid D80 with reduced outer ends)

 

On 2500 SRW trucks both axles use the same calipers, rotors and pads as far as I know.  The DRW are different as is the axle housing.