Uncle John's Band - scale(s) for the riffs
 

I know the key of the song is G, but what scales other than G major are workable for this song? Our other guitarist will do the rhythm so I've been asked to learn the lead fills. I first tried mixolydian but couldn't tell if that was correct. I do know that the Dm pentatonic over ending works well but can't find much discussion for the fills during the song. Suggestions welcome. Thanks!

Honestly, I play whatever sounds good to me, then figure out the labels later...if ever.
The common modes all use the same set of scalar intervals. What they're called depends on where you start.

The D minor pentatonic would have an F natural. Most of what I hear Jerry Garcia playing is from a G major scale and that has F#. If you are nervous about using the full major scale you could experiment with the major pentatonic.

Jerry is pretty much living in G major on this one with the single exception of using the #4 passing tone (C# in the key of G) on the way up to the D in a few places (bar 2, beat 3 of the main solo on the studio recording is eighth notes B C C# D). But otherwise he's solidly in G major.

Time for my two favorite Dead jokes:

Q. Why is their music like spaghetti?
A. It's a bunch of long noodles.

Q. What does a Deadhead say when the pot runs out?
A. "What's that noise?"

Now back to your regularly scheduled program...

Check out this site foe Dead tabs, including UJB

Rukind.com

More Dead Jokes...
 

Q: Why do Deadheads dance while waving their hands in front of their faces?

A: To keep the music out of their eyes...

(This one was told to Jerry by a soundtech friend of mine)

Q: How many Deadheads does it take to change a lightbulb?

A: Only one, but it takes 10,000 to follow that burned-out bulb around the country...

(Apparently, Garcia loved this one, laughed 'til he cried...)

Jerry was probably thinking chord shapes...he played changes.

This thread is 7 years old, but if anyone else finds it through a google search, maybe my contribution will help.

As stated here & elsewhere, Jerry plays G major over the part of the song with lyrics, but a lot (most?) of his soloing on this tune is in D mixolydian (the notes of G major, played from D through D).

Jerry switches to D Dorian over the instrumental part at the end of the song (comprised of alternating 4:4 measures of Dmin and 3:4 measures of C, G, D2). Dorian is like minor and mixo grafted together; starting with Whole step, Half step, Whole Step, Whole Step, ending with Whole Step, Half Step, Whole Step. It can sound like either minor or major, depending on what notes you emphasize. It's a great scale to get creative with.