The modern world is coming to this isolated Nile River town deep in southern Sudan in strange bits and pieces that do not quite add up yet to the much-advertised "new Juba."

Imagine a town of 130,000 that has no telephone communications with the country's capital but has just gotten color television, and where a sleek Boeing 737 arrives daily from the outside world but local cars and trucks are often immobilized for weeks because there is no gasoline.

This is Juba, the capital of the "new south" that is emerging from the ruins of a 17-year-long civil war in Sudan. Noticeable sleepy, it resembes nothing so much as an overgrown. African village, with thatched huts interspersed among brick or cement-block homes, and animals grazing all over the place.

Nearly everyone here spends much time and energy seeking shade from the devastating heat and sipping beer, lemonade or tea at office desks and in small subbyhole bars. A favorite hangout of local officials is the mango tree-shaded veranda of the town's one hotel, dubbed by outsiders the "Juba Hilton," where expatriates and Arabs from the north alike curse the food and service with unquestionable sincerity."

BUT JUBA IS NOT without a growing number of assets, far more appreciated by long-suffering residents than by irritable visitors. There is a Dutch-built bridge across the Nile now, replacing an antiquated ferry-boat, and fully 17 miles of asphalt road, the first in the south.

A $5 million, Yugoslav-built government complex comprising a Parliament, 11 ministry buildings and 28 officials residences form in effect the nucleus of the New Juba, and nearby a new university is about to open with 100 students.

Before long, the regional capital will have telephone, Telex, radio and television communications with both the north and the outside-world, thanks to the American-built communications station going up on the outskirts of the town, which will cut down by light years the time needed to make contact even with Khartoum Television," in living color" even, has just begun, although few residents other than top officials have sets.

Right now, most communications out of here is by Morse code over a shaky radio beam to Khartoum. The United Nations has the only radio-telephone link to the capital.

In early March, as part of the celebrations to recall the accord that ended the civil war five years ago, the cornerstones of the new Ordeon Theatre and the A to Z Supermarket were laid with appropriate fanfare. A culture center financed by Arab contributions is also going up.

Still, Juba lacks some pretty basic items: There is not a single functioning gas station anywhere in town, because of the chronic shortage of gasoline. Dairy products are a rarity - there is a new dairy and poultry farm eight miles outside town, but the road is so bad that the eggs being transported into Juba were mostly scrambled by the time they reached market.

Often, there is simply nothing to drink but lemonade, because the delivery system is precarious and irregular. But Jubans are discovering that Mombassa, Kenya's port on the Indian Ocean, is far closer than Khartoum - and far more certain for getting goods in.

Still, Juba has prevailed over all these shortcoming and is slowly progressing toward a far brighter future than it was ever known in the past.

"It's a thousand times better than it was four years ago," remarked a United Nations official. "The Nile bridge and the Boeing alone have made an enormous difference.

A VISITOR to Juba can be left with little doubt that southern Sudan belongs to the world fo black Africa and follows a different drum from that heard in the Arab north. Seven-foot-tall Dinkas and Nuter and Anuark tribesmen with their marked facial scars run the regional government here and give the town a distinctively African character.

Life moves a good deal more slowly on the banks of the Nile here than even in the north, and southerners do not like being rushed or ordered about by Arab northerners.

But neither the Aran northerners nor the many expatriates working here seem to have done much to help train southerners to run their own affairs more efficiently. Arabs still run the airport, telecommunications and other vital services and have a monopoly on the retail business, running most of the shops in town.

The southerners seem extraordinarily tolerant about this state of affairs, at least until provokes or teased by a northerner. At that point, however, one has the impression the civil war may break out again on the spot. But southern officials seem well aware of the south's extreme limitation and enormous dependency on the north for just about everything.

"They're always asking us for technicians and adversers," remarked one Arab official. "What can we do?" said Regional Information Minister Madingde Garang. "We advertise for new people for our local radio and press, but nobody comes. There is just nobody around her to fill such jobs."