Can I rebuild my 8 3/4" myself?

You want 4" or 4.5" bolt circle unless you have some odd wheels.

You can get a complete pumpkin ready to go from Dr Diff and axles are $300ish. No way should it be $2100+ unless someone else is installing it.

I thought so too. But I looked at Dr Diff and it was dropout $1375, Axles $295, Brakes $300, gaskets $25. Pretty close to $2100.