Kenmore 417 laundry center - how do I get the tub off the transmission to replace the seal?

You don't need to pull the transmission for this one. The seal is buried at the bottom of the tub hub, right where the shaft comes through, and it's hidden under a trunion, which is that metal block sitting at the base of the shaft. There's a retainer plate clamped across it with a couple of bolts accessed from the side. Pull those bolts, slide the retainer off, and the seal is right there underneath. Once that's off, the tub will lift up and off the shaft, though it's usally stuck on with years of detergent gunk, so work it back and forth and don't pry against the tub rim. That's where people crack them.

Before you button it back up, check the shaft itself while you've got it exposed. If it's scored or rusted where the seal rides, a new seal alone won't hold and it'll weep again in a few months. Clean the shaft up with some fine scotch brite and make sure the seal surface is smooth. Grab the seal kit rather than just the seal, since the kit usually comes with the retainer hardware and you'll want fresh parts there anyway.

As for whether it's worth it, absolutely. I did this exact job on a Frigidaire-built stacked unit a few summers back and it's maybe an afternoon of work once you're already torn down like you are. These older laundry centers are about as simple as washers get, direct mechanical stuff, nothing fancy to fail, and the parts are cheap. I'd take one of these over the newer units all day. A friend of mine bought a current version that looks the same and it's been nothing but trouble.

One tip from my repair: take a phone photo of the retainer orientation before you remove it, so you get the bolts back in the right spots. And when you reassemble, a little dab of grease on the shaft where the seal slides on keeps it from folding or tearing the lip on the way down.

Did one of these last year on my mom's unit and the thing that got me was the drain hose fitting inside the cabinet. Had the same drip symptom and figured it was the seal, turned out the hose had worked loose at the clamp and was spraying just enough to run down and pool in the same spot. Check that before you order parts, it costs nothing to look. Mine was actualy both, so glad I found the hose first. Once the new seal went in it's been dry ever since, two years now and counting.

Done a couple of these Frigidaire-built stacks and the one thing that bites people on reassembly is getting the tub seated back down square on the transmission. If it's cocked even a little, the shaft drags the new seal sideways on the way down and you're doing the job twice. I put a level across the top of the tub on my second one and caught it tilted maybe a quarter inch before I torqued anything down. Rock the tub gently till it drops evenly, then tighten the retainer bolts a little at a time in a crisscross pattern so it pulls down flat. Also budget a new agitator coupling while you're in there, mine snapped three weeks after I put it all back togather.

Update: first checked that drain hose fitting inside the cabinet like one of you said, and it's dry and tight, so the seal really is the problem. Good news is I found the trunion you mentioned, retainer plate and all, just needed to look lower than I was. One more thing before I order the kit, does the new seal press in dry or should I hit it with a dab of grease first?