Without pulling the chunks or driving it, about the only thing you can do is get underneath and turn the pinion flange by hand with the wheels off the ground and see how it feels. It should feel smooth by hand. Also make sure you don't have any slop or play in the pinion. It should feel tight with no movement in or out, or up and down. You can also drain the fluid and see what it looks like. Swamp lube = stay away because they will likely need a full rebuild. Nice clean fluid is what you are looking for. When you do get them out then you should check the backlash and preload on the pinion and carrier to make sure they are good.
I think the front diff is pretty standard as far as how it goes together OE vs Aftermarket, but the rear is not. The OE 4.88 is a dealer only setup, so the carrier, ring, pinion, etc are different than an aftermarket setup. I think the carrier has a thicker lip and the ring is much thinner. Aftermarket gears get very thick in order to account for the smaller pinions.
I would love to find a set of factory 4.88s. If you don't mind me asking, what is your reasoning for the 4.88s with 33's? I was actually right in the middle of doing some research on this very thing. I have a 90 that I am looking at building a front axle to swap into at some point and trying to decide what gears to put into the HP front that I have for it. Probably running 33s maybe 35s. Not a wheeler, will be a driver and a farm truck. Would appreciate your input.