i've been trying to figure this out for the past couple of hours. online converters are pretty useless.
printed off a couple fret board layouts for ukulele and guitar. i've been going note for note but the octaves get in the way and throw me back to the start pretty much. there is no pattern i can find. i wish i knew more about music or muisc theory so i could actually get somewhere. i've found translating guitar to uke is a common problem. someone should figure this s** out because i'm having the hardest time. but i'll try all night until i get it
To a point, it depends on what tuning your uke is in. If you're playing in D-G-B-E, the part of the tab that's on the top four strings of the guitar should be the same as on the uke.
If you're playing in G-C-E-A, you can still use the same tab for the uke (for the top four strings, of course) if you don't mind changing keys - if you use the same tab for a song that's in D on a guitar, it'll be played in G on the uke. The relationships between the notes will stay the same, though.
It gets a little more complicated if you're using high G tuning. You can still use the part of the tab that's on the top three strings of the guitar, but as you say the fact that the G string is an octave higher will make things a little strange. It's probably not a huge deal for chords, but if you have single-note lines in the guitar tab that run down to the fourth string... well, you'll be out of the uke's range. There are options: you can move the line up an octave (and still play it on the G string, so it's easy to tab out) but it'll probably sound weird. Or you can do what sharp21 suggests and transpose the whole thing so that you're not out of the uke's range anymore.
If it's important that the song be in the same key on the uke as it is in the original guitar tab, you'll have to do some transposing. That's going to be a little tricky, there's really no getting around it. However, there is a pattern that you can use to your advantage:
If you capo a guitar at the fifth fret, you have a ukulele tuning (G-C-E-A). So you can take your guitar tab and subtract 5 from whatever the fret number is; that'll give you the equivalent fret on the uke (for the top four strings)(unless you're playing reentrant; then this will only work for the guitar's top three strings). If this gives you a negative number, you move to the next lower string, using the fourth fret for "-1", the third for "-2", the second for "-3" and the first for "-4". For "-5", you play the next lower string open. HOWEVER, when you're moving from the B string of the guitar to the C string of the uke, use the third fret for "-1", the second fret for "-2", the first fret for "-3" and play the open string for "-4." The open B on the guitar would actually be played on the fourth fret of the G string on the uke (unless, again, your uke uses high-G tuning; in that case, it's not going to be available). Do this for every fret noted in the tab, and you'll have converted the guitar tab to ukulele.
The big problem with this is going to be that you'll run out of room on your ukulele fretboard, because the guitar has a bigger range (particularly if you're using reentrant tuning). There are options, but they're going to be sort of problematic. You can always move things up an octave, but won't sound like the original song (could be a good thing; could be a bad thing). That's the only way (AFAIK) to preserve parts of the tab that are played on the guitar's low E and A strings, and if you're tuned reentrant you'll need to do something similar to anything played on the guitar's D string (and the top five notes on the G string) too.
The other thing to do, which is admittedly a lot more time consuming up front, is to convert guitar tab to standard notation (or, if you can, just find the standard notation; sometimes you'll see both), then convert the standard notation into uke tab. If you do that a few times, you probably won't even need the uke tab anymore.