Doubt you'll find a complete disc front end for $50
You could try your local pick-a-part as they are relatively cheap if you don't mind pulling it yourself. Finding a complete dana 44 front isn't all that hard to do. Disc became standard on these things starting in 72.
Even $150-$200 would be cheap in my opinion for a complete front diff with disc, and a worthwhile swap. I recently bought front and rear diffs from a 78 complete with 3.73's, posi, all new front suspension pieces complete for $500. They are out there.