I think an '87 should have a transfer case lever... and automatic (vacuum powered locking) hubs right? Pretty sure that if that's the case you have to be in park or nuetral and not moving, then shift the transfer case into a gear (should have 4 low, nuetral, 2 high, and 4 high from closest to the dash to closest to the seat), then shift the transmission into a gear and you should be good. If you shift from 2wd to 4wd it's instantaneous. If you go from 4wd to 2wd you need to back up a foot or two to disengage the hubs. My '87 is a one ton and has manual hubs, but everything else is the same.