OK do you have a lift? and how much?
With a lift, you don't want it to be all the way in because if you stuff the rear, like when you jump or you go a little too fast where there is a dip, you will crack the case.
Now too much out is also not good.
If I recall right, the slip yoke is a total of 6"-8"
I would say 1/4 of it's length outside at rest is OK as long as there is enough length inside to keep it rigid and not wedge sideways.
If it were me I would take it out and check.
Also if you can, lift the truck as far as you can and let the axle droop, and see how much more it goes out. If at full droop you still have at least half of it inside you are OK, if not, you might have a problem, and you need to lengthen the driveshaft.