I tried the driveshaft thing and it didn't work out for whatever reason.. I did that many times at yards the past month or two when searching for a donor rear end for my truck and the wife's. Internet suggested the two wheel rotation deal and that was most accurate when comparing those results to the RPO code in the respective truck's glove box. Supposedly I put a G80 3.42 in her truck two weeks ago, the two wheel rotation thing showed 3.5ish turns of the pinion though. So that either speaks to the accuracy of the method or I've got some GM truck differential ratio GM never made..
Clutch type limited slips behave like you mentioned but the GovBomb differentials will behave like an open diff with both wheels off the ground. You either have to start the truck & put it in gear to find out if it's a 'bomb or pull the cover off like I did at a yard. Oh and I think ya meant "open diff" and not full floater