Jack the truck up from the frame, so the suspension is fully extended and see where the upper mount fits on the frame. Also check where full compression would be with the top mount in that position. You can measure from the bumper to the frame to see where full compression would be, or you could use a jack and chain to fully compress the springs. Be careful if you try that as the jack may come flying out if not secured.
[on edit]On the AD series trucks that used tubular shocks, the rear top bracket is fastened to the frame with only 3 of the 4 holes.
