Housing width is the only difference I am aware of. The actual guts of the differential should be the same. 99 - up trucks use a 8.6" ring gear and the carrier requires different bearings than the 8.5" ring gear used prior to the "NBS" GMT800 trucks. In other words, you could use the parts from a 99 - up in your OBS, but you would need different carrier bearings.
You didn't mention what you have a for a truck, but I thought I'd put it out there.
Oh yeah, and 1988 OBS trucks were 28 spline, not 30 spline, so those really won't interchange with much unless you swap axles.