Originally Posted by xenon
If the doughnut mounting holes have worn in an eccentic pattern with elongated holes, then at lower speeds or when the propshaft is unloaded, it can reciprocate around the logitudinal axis causing the side-to-side "feeling". At higher speeds or under load, the spinning smooths the imbalance gyroscopically. The only real way of telling is disassembly.
brilliant explanation and sounds to make perfect sense - it feels fine at high speeds, but 'swing-ey' (not a real word I know) at low speeds.
I will get it on the ramps and check.
Is it a big job to change - I have the use of ramps and tools etc and I'm a confident DIY'er.