As Rebal said...run away and fast....We could help here way better than any dealer...

 

If the mechanic found "play" at the yoke on the diff, there are a handful of things that can cause this, and most do not require an "overhaul".

First would be a loose pinion nut. Not common on these axles if I understand right, but maybe. Cinch up.

Second would be too much shim on the pinion, unless it uses a crush sleeve, but I understand these don't use them.

These things would negate the need for a "overhaul". When you need an "overhaul" is when you see grey crud in the gear oil indicating bearing failure.

I almost wonder if the mechanic is a dipsquat and misidentified the "slop". Rotational slop is REQUIRED in ring and pinion setup. If you have no backlash that creates this slop your gears will get ruined. Lateral or linear play indicates loose pinion bearings.

If the bearings get ruined, slop can show up, but again grey mush in the oil. Otherwise, an improperly installed outer race, loose pinion nut, or too many shims on the preload spacer would be the problem.

This slop can break the hell out of ring and pinion gears if excessive because it allows the gears to bind up, and will cause irregular wear on the gears if even minimally sloppy. The worst breakage/wear is in reverse because when going forward, the pinion head is pushed back into place by the gear deflection.

I'd give a more coherent rant, but spent all day dragging 2.5" hose around.

 

The only thing that would require complete rebuild is if the gears (ring & Pinion) are howling or damaged, pinion bearings usually get pitted and the bearing race for the carrier have spun in the housing. Usually if that happens you would have excessive backlash and end play in the carrier. That would add up to a complete diff overhaul. Usually the magnet would look like a chia pet, the diff fluid would have metal particles and smell burnt. Dana 80's are bullet proof I say that because I've only had to rebuild a few in 25 years. I mostly see pinion seals leaking on the 80's. Now if we were talking about a 9 1/4 on the Ram 1500 trucks, then I can say I've rebuilt my fair share. If you decide to go for a used one, then I would have someone pull the cover to check out the above first. Could save you time and money in the long run.