Deore XT is a mountian bike derailleur.
105 is a road bike derailleur.
Similar quality but the XT will handle a larger rear cog than the 105.
If road bikes hurt your back it's time to do some stretching and get your back into shape.
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The Lord is merciful and gracious, slow to anger and abounding in steadfast love. - Psalm 103:8
I am a cyclist. I am not the fastest or the fittest. But I will get to where I'm going with a smile on my face.