、。
She tore his letter to shreds in tears and threw it away.
Literal
She [topic-は] crying [while-ながら], him [from-からの] letter [object-を] to-bits ripped-apart [and-て] threw-away.
The compound particle ~からの compresses 'from X' into an attributive phrase that can modify a noun: 彼からの手紙 means 'a letter from him' as a single nominal unit. Bare から alone can't modify nouns like that; からの is the bridge. むちゃくちゃに is the adverbial form of the noun/na-adjective むちゃくちゃ ('mess, chaos'), giving 'in a chaotic / haphazard manner' — here, ripping the letter wildly into pieces. 引き裂く is itself a compound: 引く ('pull') + 裂く ('split, tear'), with the 引き- prefix intensifying the tearing motion. The te-form 引き裂いて chains into 捨てた, sequencing the two violent acts.