Thanks very much for your collective advice. I’m going to go ahead and replace the pinion seal and see what happens - it is worth a shot for $100, guarantee or no.
As far as the overhaul of the diff - his quote was based on the diff out of the car. There’s a spare 2.88 diff kicking around in my shop and my intention was to have him build my spare diff and I’d R&R the IRS to keep the downtime reasonable.
For him to take the diff out of the car and replace it after the diff was overhauled would be another 9.6 hours of labor on top of the 3.5 hours to setup the diff after replacing the seals.
I suspect that if I were to let him do the whole job, I’d have a bill for north of $2000 for parts and labor.