Unless you get a known like-new "donor" unit, or if you get a "donor" with any questions whatsoever about its lineage, it may be worthwhile to go through it before making the swap.
The 4L60E is a fairly easy rebuild, and is reasonable, even with premium parts:
http://www.wwdsltd.com/4L60/4L60ERepairs.html
For your applicaton, only 1993 & 1994 units are direct drop-ins, and probably none of those are going to be low-mileage, pristine units.