Swapping the whole rearend would be far easier. You can easily do it yorself with a pair of jackstands, a floor jack, and a day of labor. The hardest part is bleeding the brakes and even that is easy.
Swapping gears is not usually a job for a novice, since the clearances must be very tight in a rearend, and if they are off by to much, it will self destruct.
However, i have had poor luck with used junkyard rears. I put one in my truck a few weeks ago. I inspected it well at the junkyard, pulled off the cover, and all looked well (no metal flakes in the fluid, no play in the pinion bearing, gears looked great). A week after the swap it started howling, and now i need another.
Getting your rear rebuilt is a surefire way to know you will have a relialbe rearend.....but you may get lucky with a used rearend. Remember, they are all over 30 years old and most have several hundred thousand miles on them.
BTW, swapping rears is much cheaper....$150 for a used rear, and $25 for new U-bolts and you should be set
