Jack car up UNDER REAR AXLE TUBES with driveshaft installed. Make sure it can't roll on you. Mark or measure where installed driveshaft stub goes in trans right at the seal. Then pull back of driveshaft loose from rear end and then take shaft and slide it further in (too far) in back of trans till it hits softly and measure again and determine the difference in the two numbers. You need one inch firm to allow for in/out as the suspension works in normal use and that also guarantees the stub is in far enough. If less than one inch and you hit a really big bump or pothole, stub will go in far enough to knock out the stub end plug and kiss all your ATX fluid goodbye, you find out when trans locks up from no fluid in it.