One could create an entire D&D World with all the magic removed. One can go through the monster manual and remove all the magical creatures from consideration and those that are left can go to create the encounter tables for that world. You can also Travellerize the D&D classes by changing their hit points to stamina and giving them life blood scores. These D&D classes can be substituted for the Traveller Barbarian class. There is the D&D Barbarian, The D&D Fighter, and the D&D Rogue which can be used with only this basic modification.
The Bard Class would have no spells, some of his other abilities border on magic so its an open question whether to eliminate them.
The Cleric class could exist, but you'd have to eliminate his ability to use spells. I'd leave his ability to turn undead and create holy water though, because in any campaign where undead actually appear there is equal justification for the Cleric to have these abilities.
Druids also have their spell use and magic abilities removed, though they can still exist.
The Traveller game has no Monk class though there is no reason why one couldn't exist. Much of the Monk's abilities come from inside himself, since this is similar in many ways to Psionics and Traveller has Psionics, then the Monk class can be left mostly unaltered. The main exception would be the Perfect self which he achieves at 20th level where he becomes an extraplanar being.
A Paladin is a great thing to have in a Traveller campaign. Spell use is out of course, but his defensive abilities could be left in. Divine health can be explained away as a robust immune system. A Paladin can detect evil, know one knows how, perhaps it is instinct or an innate sixth sense, or perhaps its relating to some Psionic ability that he was born with. Divine grace allows the paladin to apply his charisma bonus to saving throws, magic use can't be proven of course, to other characters he just appears lucky. Lay on Hands might just double or triple a character's natural healing rate over the next several days rather than heal someone immediately, it could be chalked up to chance. An Aura of Courage can be considered a charisma effect. Remove Disease simply aids the recipient in recovering from a disease over the next several days. Smite evil works fine against characters who are obviously evil, no connection to the divine can be proved, and the shots just seem to be particularly well placed. If for any reason Undead show up then the Paladin can turn them as both phenominon balance out. Turn undead doesn't manifest if there are no undead around. Extra turning likewise, No Spells though. The paladin keeps his ability to summon a special mount, whatever good that does in a high tech campaign, and of course a Paladin has his code of conduct that he must uphold. The Traveller Universe is not an ethically black and white Universe, it has alot of shades of grey, but the Paladin sees things in black and white as a constant struggle of good versus evil. Paladins don't have to come from TL 1 societies either.
The Ranger gets a similar treatment to which I gave the Paladin.
A Sorcerer and a wizard is a Charlatin or a stage performer. These two classes are very skilled at using high technology and fooling primitives that they actually have magic powers or spells. They can construct laser pistols that look like magic wands for instance, they have crystal balls that are actually electronic holographic displays. A magic carpet and a flying broomstick are actually cleverly disgused grav vehicles.