After Vader's battle with Luke, he was in a very bad way. One hand was gone, he was tired, and he had suffered under Luke's onslaught.
Darth Sidious, on the other hand, was at his full strength. He was in full control of the Dark Side, had no injuries, and could unleash everything at Luke.
If Vader had tried to attack using the Force, Sidious would have responded, and they would have at least been equal in strength. A Force-choke takes time - more than enough time for Sidious to easily break Vader's grip. Given Vader's condition, Palpatine would have won a Force battle.
Despite Palpatine's powers of the Dark Side, he was still a physically old man; approximately in his late eighties.
Vader knew that his only chance was a physical attack. In terms of physical strength, he was the superior. So his lifting Palpatine above his head was swift, and took the Emperor by surprise, giving Vader enough time to throw his former master down the shaft, even though it cost him his life. In that moment, it was the only way he could see to save his son.
From the official novelization:
At that instant, Vader sprang up and grabbed the Emperor from behind, pinning Palpatine's upper arms to his torso. Weaker than he'd ever been, Vader had lain these last few minutes, focusing his every fiber of being on this one, concentrated act - the only action possible; his last, if he failed. Ignoring pain, ignoring his shame and his weaknesses, ignoring the bone-crushing noise in his head, he focused solely and sightlessly on his will - his will to defeat the evil embodied in the Emperor.
from Star Wars : Return of the Jedi by James Kahn