I just had similar issues.
Firstly, IMHO, steering wobble is more likely balance, alignment or maybe a bent front wheel. Under your butt, more likely ujoints, center bearing, rear wheel balance, or bent wheel.
My issue was dead ujoints and sagging center bearing. First symptom was vibration on aggressive acceleration (center bearing). Second symptom was a "click" on startup (ujoint). Third symptom, "crunching" on aggresive start in first gear (ujoints I believe). That was when I'd had enough. I removed driveshaft and sure enough, ujoints and center bearing were toast.
Had new ujoints peened-in, center bearing replaced, and driveshaft balanced ($400). I re-assembled, pre-loaded center bearing, and went for a test drive. I believe I still have a slight bit of vibration under my butt on aggressive acceleration, so I’m going to make an alignment tool and confirm that my driveline is straight.
I, as well, am not sure how to easily re-align driveshaft as there really isn’t any adjustment at the center bearing, unless it’s to be lowered. Then you can add washers/shims. It's never that easy.