I've heard arguments for both sides, and I would lean towards shackle to the rear. The on-road ride should be better and performance off-road should be better too.

The only thing I can think of that would be a positive for the forward shackle is it provides better driveshaft geometry during full travel of the suspension.

Pay attention to your slip on your shaft and I would consider that a non-issue.

Rear mounted shackle has my vote.

'06 Dakota QC 4.7 6sp 4x4
'88 Mighty Max
'77 KZ1000 fun old bike