it'll bolt up fine, the issue is the '87 front drive shaft will be too long, and the transmission on the '90 is most likely a different length (unless the '87 has an A4LD), then there's the '90 having a 1354 transfer case instead of the 1350 that's in the '87 which means a different style front drive shaft, if they are both auto or manual and the transmission is the same length you can just swap transfer cases and use the '90 front drive shaft. Also note that the front and rear axle gear ratios need to match, but the '90 rear axle should be a bolt in for the '87, might have to swap U joint yokes on the rear of the drive shaft but other than that bolt in.
In short, with the right combo of parts it'll work just fine, my entire drivetrain is assembled from unmodified (well, made better than stock but not shortened or anything) stock parts