If you could do this for $4K, it might be more of a "sure thing" if there is such a thing!
As Christo mentions, there are a lot of things you would want to do "while your there". Maintenance items that would really make sense to do with the engine out of the way. The vehicle is still nearly 20 years old and has a lot of miles on the original vehicle. I would want to know if the radiator has been changed with a quality radiator? Fan clutch? All hoses? Front oil pump seal on the newer engine? Plugs, wires, air intake hose, etc, etc, etc. All of these things add up very quickly to another $1K or so and that doesn't even account for any axle maintenance that needs to be done.
So, for $4K...I would feel better about it.
Lastly on the lockers....the resale for an 80 with lockers in CO is about $3K more than vehicles without. BTW...it costs a bit more than $3K to add the ARBs and change the gears (unless you can do every bit of it yourself). I just did it, so I know it's not cheap.
Horribly Painted Moonglow '95 FZJ80 Has Some - Still Needs More!
White 2008 GX470 - Nearly Stock
"I drive an ECO-FRIENDLY FZJ80...It's got a salvage title which makes it a REPURPOSED/REUSED/RECYCLED vehicle. Therefore, don't mess with me when I park in the "Reserved for Green Vehicle" spots in Boulder!!!