That sounds bad! They rounded them off or your did trying? I can't imagine using any lock tight on those. Really not under much pressure. I have had mine off before on the front diff and filled it myself when I had a leak. (leak was fixed by mechanic shortly there after). Sorry to hear about this. I have not had any issues with bolts/nuts on my car and find the materials used for such to be great for long life. Hope it works out for you without some huge bill.

Mike