I always find it much easier to find my pitch while I'm playing the piano than while playing the uke. This confuses me to no end. Do any of you experience similar things, finding it easier to sing on key while playing some instruments versus others?
Also, I was thinking, if you have a low singing voice, you might be "tuning in" to the bassier sounds on a piano to get your singing key, but you're not finding that on your uke. I'd assume that practice would make that perfect. Perhaps even practicing getting your key by going to the piano, then the uke, then singing it. It might help re-program your brain to look for your starting note using different cues.
While we're at it, its the INTROs and OUTROs that can really make a difference.
While practicing your favorite songs, just go ahead and work out an INTRO (if you're not too imaginative take the last line in the song as the Intro)
and an ending (may need to be more creative here, or just repeat the last line and ritard/slow down)
Anyway, all that to say that playing an INTRO can also help in recognizing and aligning with the proper notes to start the song strongly
Not quite what you're looking for, but when I took voice lessons, we'd warm up on the piano but she would accompany me otherwise on a guitar because it matched my voice much better.