Play in rear end

[Rifter_Chevy]

Ok i know that title sounded bad but i think i have a serious problem.

My truck makes on hell of a loud thumping/clunking sound when i put it into reverse, and then another really loud thumping/clunking when put back into drive after moving in reverse.

I had a buddy drive it back and forth while i watched from underneath(yay for hanging from the frame while someone drives your truck around a parking lot) and found that the sound is 100% no doubt in my mind coming from inside the rear diff.

The issue is there is 2-3" of play in the driveshaft BEFORE the tires start to move. For example say i was going forward, then i stop, then i put it into reverse, the thump comes because the driveshaft and pinion move really really fast for those 2-3" of play then makes hard contact with the gears inside the diff and then tires start to move and makes a really loud thumping sound.

How do i fix this? is this backlash related or pinion depth related?

EDIT to add, truck is a 4x4, doesnt do when 4x4 is engaged im assuming because my front diff stops rear shaft from closing that 2-3" of play. Also it is a 14 bolt Full floater rear end.