Truck Repair: 1999 chev tracker, loud thump, manifold vacuum

Question
I need help with the 4x4 i hear a loud noise only in 4x4 sometimes going straight one loud thump if i turn left in a circle it will pop at least once every circle,i have taken out left axle with cv joint all looks good,could it be the vacume line that conects to front diff, could you tell me how much vacume it takes to work the 4x4 or how to see if the part that ingages in the diff.is working.  thank you i appreciate any help.

Answer
Hi David,
I don't think I can help much. I have no way of knowing where the pop is.
You also have a transfer case where the problem could be.
As for vacuum, It uses manifold vacuum.
One way to know if it is engaging is to jack up the right front tire, and in two wheel drive, you should be able to rotate it, but then engage four wheel drive, and you shouldn't if it engages.
I would try to get all four wheels blocked off the ground, and then see what it sounds like.
Van