I'd check the fluid level first and then flush out the tranny fluid system a bit and see if this cures it first as it often has for me. Also maybe the tranny linkage is buggered-up and just needs a slight adjustment to engage reverse.. Indy says it needs a rebuild but do they have any idea why without cracking it open ? Maybe it's best to get 2nd opinion from dealer first and there may be an easy fix the dealer would see... either way - correct diagnosis before any money changes hands is the most important part of wrenching !