Newbie completely at a loss.

whatever you get,
since its going on a spring piston/gas ram gun make absolute certain its "airgun rated" or its very likely you will destroy it in short order.
break barrels are very hard on even high dollar scopes if they arent made to handle the unique recoil of spring guns. 

It depends a lot on your budget, what are you looking to spend? I really like the new Airmax that I got. It is a 6-24X50 and focuses down to 10 yards. You can use any target scope to hunt with, but if you want to be able to shoot long distances accurately without rezeroing the gun every time you change I would suggest getting a Mil Dot or MOA scope so you can use the reticle for hold over as well as ranging. Im not familiar with the Yukon but I think its a break barrel, in which case you need to consider the recoil when selecting a scope also.

http://www.airgunnation.com/topic/list-of-scopes-suitable-for-air-guns/ 
There is a post with a spreadsheet showing what good scopes are available on the market I would read this thread it will help.
I personally like the Sun Optics Air Tac, 3x9x50

Leapers UTG 30MM 3-12X44 Compact IE Scope w/Glass Reticle run about $160.00
I have two, they are a great scope for the price and it comes with mounts for a piccatiny rail
https://www.precisionairgunsandsupplies.com/ProductDetails.asp?ProductCode=scp3-ugm312aoiew

The Nikon 3x9 Prostaff is a perfect scope for your Ruger. Just the right size and will work up close or way out. Hunted prairie dogs with my 22 from 20 feet to 200 yards, so much fun and the Nikon is still going strong only now it is on and airgun. They are quite inexpensive at $189.00 on AoA web sight or Amazon. Not to heavy or bulky and just a great unit.

Ground hogs are great eating. 

Hammers 3-9 x 40 AO from Amazon at 75.00. Springer gun rated, AO, 

These guys are all pointing you in the right direction. "Airgun rated" is a MUST for springer rifles. 
We're all here to help each other out. So, if selecting a scope gets to be too much, just reach out to one of us that lives in your area and we'll gladly come by to assist with the ground hog "relocation".  
Happy hunting. 
Tom

No experience with the Ruger Yukon, however a few comments anyway.

"I need to permanently relocate ground hogs in my pastures"
Ground hogs are rather tough critters to take down but I've found that a pellet from a 14fpe .177 cal springer placed between eye and ear will do the job.......


"I was looking at a Nikon Prostaff Target EFR 3-9×40 that seemed to do the job and suddenly I am reading that it is a “Target scope?"
LOL.....the scope in the pic above is an old 3-12x40 Bushnell Trophy set at 12x. Ya know, any scope is a "target scope" depending on the target! LOL. In my powder burner days I could shoot sub 1"ctc groups at 100yards with a fixed 4x on my .243 Winchester but for my HW95 springer I prefer a 4-12x variable finding that it's satisfactory for all my airgunning from close range plinking to shooting hunter class field target matches. As others have mentioned, the scope needs to be "hardened" for the two way recoil of a piston gun, especially with the gas ram power plane because they have a harsher shot cycle than a spring powered gun of the same power output. Another consideration is the warranty since you'll probably need some warranty work done after a while with a magnum piston gun. Hawke comes to mind when I'm thinking of a scope that has good customer service backing.

"I also would need to know what mounts to use on that Yukon, it came with a piccatiny rail."
Even though my HW95 has a dovetail scope rail I use a Weaver to dovetail adapter between my scope mount and the receiver because I find the Weaver mount to be real convienent. Here is my HW95 with 4-12x50 Hawke Vantage scope clamped to a rather cheap set of Weaver high mounts that cost only about $10. This Weaver mount is only available for 1" tube scopes but it does work nicely. The medium height will work with scopes with 40mm objectives so I needed a high Weaver mount for my Vantage scope with a 50mm objective........




Hope this helps more than confuses!