The CV Boot on my left rear axle leaked out most of the grease. Can the boot be repacked and will the seal hold or does the boot need to be replaced?

I think you need new boots. And if one side has gone, might as well do the pair at the same time.

If it's caught early there should be a two piece repair kit available. The kit works fine.

Remove, clean, repack, and reboot axle with a one-piece boot. This is the professional way of doing it and nothing else will last, unless you install a complete new axle.

Don't waste your money on a two-piece boot. Either replace with a one-piece boot or throw in a rebuilt axle. Anything else is just a temporary fix.