Dangers of connecting ground to neutral?
Based on further conversation.... this is an emergent condition that is trying to kill you.  It hasn't succeeded only because the shock path to source is high-impedance and limiting current flow to below 10ma.  (if you can feel it, it's at least 1ma).  Naturally occurring impedances can change dramatically, so it could turn and kill you tomorrow. 

You say your garage already has a ground wire that you've tied into.  However since it is not tied back to the house, it is a "fake ground". It makes things more dangerous.  


If a grounded device has a ground fault, that current is harmlessly shunted back to the main panel.  
If an ungrounded device has a ground fault, it electrifies the chassis of that device only. 
If a device plugged into a fake ground has a ground fault, it electrifies its chassis and every other device also plugged into that fake ground. 


You have the latter situation and it's Very Bad.   Any device in the garage could be the origin of the ground fault that is giving you these problems. 

The right solution is twofold: 


Disconnect the fake ground from your receptacle(s).  This will protect you from any rogue ground faults in other devices in the buildings.  In this situation you are better off with no ground than fake ground.  Beware: metal junction boxes and conduit can deliver "ground" to the yoke (bracket) of a device, make sure that can't happen! 
Replace that receptacle with a GFCI receptacle.  This will assure any ground faults don't kill you and confine any ground faults to just what's plugged into it.  So if only your devices are plugged into it, it's gotta be safe!   Connect only hot and neutral to the GFCI.  Mark the receptacle "no equipment ground"; it should come with some stickers.  Don't hook anything to the LOAD side of the GFCI. 


Let's go into the details in my original answer. 

Tying neutral to ground makes things much worse!

Under ideal conditions, modern electrical wiring is done as an isolated system, with a safety shield.  Without grounds, it should be the same, only without the safety shield obviously.  

What's the deal with the neutral-ground bond in the main panel?  That is only to bond the isolated system to protect it from ESD, lightning and ground faults.  Never bond neutral to ground anywhere else - if the neutral wire breaks, neutral will float up to 120V, and that means, so will ground!  We see people fry themselves this way on a regular basis. 

No appliance anywhere should be leaking any conductor to its case.  Not even neutral (as you propose) - because if you flip the plug over, or lose neutral, neutral becomes hot.  The ground shield would help if it was there.  A GFCI/RCD device would help a lot. 

Get a GFCI

A GFCI receptacle or breaker will detect ground faults for you and shut off the power.  Mostly it's useful as a diagnostic aid, "when I touch X, the GFCI trips" which tells you X has a ground fault.  The GFCI doesn't care whether your building has grounding; it is comparing hot and neutral current flows and looking for current that has gone astray (e.g. through you). 

Write yourself a get-out-of-jail-free card

Since you are leaving this behind, make sure you write a letter to the landlord advising him of defects in the garage wiring and the fact that you experienced tingling, shocks, whatever you did in fact experience, and that you did make an effort to reduce the hazard but that fixing it properly was impossible for you.  Keep a copy of the letter, it is your get-out-of-jail-free card if anyone is hurt in the future.  

If someone is hurt, the landlord's best defense is "I didn't know" and to pin it on anyone but himself. His lawyer will force him to viciously deny receiving the letter; he will claim you created the hazard.  You want as much "paper trail" as possible to prove you sent it: at the very least a certified mail receipt, better sending a CC of the letter to the city's electrical inspector, and best having your own lawyer send it.



The rest of this advice applies to people able to modify the home at will. 

Retrofit that ground!

Many people refuse to add grounds because they think they need to replace all their wiring. Now let's pause for a moment.  Wires that carry your power ordinarily (hot and neutral) need to run together.  That assures their magnetic fields cancel out and don't cause heating that starts a fire.  However, this does not apply to ground wires.

So in the 2014 NEC, Code was revised to give you wide freedom to retrofit grounds.  So you can just run the ground wire you need any way that is reasonable - including between buildings - and you don't need to follow the same route.  The book says "Git r done"!   And hardware stores sell bare ground wire, or you can take the insulation off Romex. 

Don't parallel wires

Noting that you converted a pair of 3-way switches into non-switches, you should not have tied the two messengers together on both ends.  Paralleling wires causes a lot of subtle problems.  You should use one messenger or the other.   The other wire should be capped off as a spare.  

Gee, what would you do with that spare?  Hum de dum...  I can't suggest using it as a ground wire because we follow Code here and there's a Code requirement that ground wires under 6 AWG be natively green or bare, and not just taped green.   

A ground rod to the building is not enough.

The grounding system has at least two purposes:  One is returning natural electricity sources (ESD, lightning) to earth.  A local ground rod on your building will do that fine.  

The other is returning fault current to source (which is where artificial electricity wants to go).  A ground rod in a remote building will not do that because dirt doesn't conduct electricity well enough to reach the ground rods in the main building.  That is why you need to have a ground wired between buildings. 

If you really need a lot of power out there

Since I assume the 3-way is a 120V circuit, you could really bump up the power out there and solve your grounding problem by putting a transformer in your outbuilding and creating a separately derived service.  A transformer's isolation moots the ground wire back to the house, and requires a local grounding system (since this would then become a main panel). 

However, this requires more education and a lot more respect for electricity than I have seen in OP. This stuff can kill you, and it's already trying.