[b-man]

Buy a new rear end cover gasket.

Pull the cover, which will of course drain out the gear oil and look at what’s going on inside while moving the wheels back and forth like you did previously.

If it’s looking bad inside there put the rear cover back on with the new gasket and refill the rear end with the old lube you drained out. Drive it or flatbed it to a shop and pay the man.

Perhaps source the new posi unit yourself so you end up with the unit of your preference and maybe a ring and pinion set if your old gear set is either questionable or you just want to change ratios.