is the rear end of a 99 the same a a 02

Look on the drivers door frame "VIN" sticker for both vehicles to see if they have the same "code". There are 3 different axles offered for those years and you need to make sure you have the same gear ratio, unless you are going to swap "just gears" or the whole center carrier (which might require axles swap as well if the splines are different then the carrier).

Will my 1996 ford ranger 4x4 rearend fit a 1999 ?

Yes

No

What is the rear end of a 1997 ford ranger called when looking for parts on rockauto?

There is the rear axle assembly if you want the whole thing or maybe you just need the differential (pumpkin). You can also buy the bearings and axles if that is what you need.