What version of the NEC are you under?

Around 2008 the 3-wire feed exception was removed, and a 4-wire feed is not required.

I don't know, where would I find out? What are the implications for my situation?

House was built in 1989, so I assume pre2008NEC.

But when you are modifying or adding to the electrical system, you have to conform to whatever code is currently being used in your area.

Brickeyee, did you mean "a 4-wire feed IS required"?

"I don't know, where would I find out? What are the implications for my situation?"

Whoever is responsible for inspections (city, county etc) in your area will gladly tell you what edition of the NEC has been adopted.

Depending on the department, the inspectors might be willing to answer specific questions directly.

Doesn't matter when the house was built. Your new work will need to correspond to the current code. If you tell us where you are, we can probably make a good guess as to NEC version (unless you're in CT, MO, HI, Reno, or Tuscon, it's probably a safe bet you're on 2008 or later).

As pointed out, the old code had an exemption to not run an equipment grounding conductor between buildings when there were no other metallic paths between the building. Your current 30A wiring appears to not be legal under any version of the code. You can't just use the ground wire in the cable for the required grounded (neutral) conductor.

I'd run the grounding conductor, it was always a good idea even under the old code.