My 3rd gen 97has 223k on the clock. Just noticed my steering wheel shaking at 60-70 mph. Slight shimmy in front wheels as well. Feels like the front end, which I think is original. Any ideas what may be causing this?

Balance, tires, age, etc. I went through this as well around 160k (our roads are piss poor in Seattle) and ended up replacing nearly everything in the front end. The culprit was almost certainly a bad UPPER ball joint, which seems a lot more rare than lower, and a royal PITA to get to, but I ended up replacing all 4 along with both inner and outer tie rods. I did a few other misc. parts, as well as new Bilsteins all around, but I'd bet the broken upper ball joint was my problem. If you have over 200k on the original lowers, take a look at those first, they can fail in a bad way.

Replaced the lower ball joints and outer tie rods at about 145k miles. The shimmy I had at 60-70 mph, which had been around for a long time and was getting worse, disappeared immediately and has not been felt since (about 10k miles).

Same thing here, pretty good steering wheel shimmy and shake around 70mph and 211K on the clock. I replaced the front struts, LBJ's and outer tie rods at the same time so I can't say for sure what the culprit was.

How does the steering rack look?

x2 - could be the bushings on the rack. Also, how's your camber/toe? Or more generally, how's your alignment? And how old are your tires? Old tires with bad wear patterns will cause bad vibrations (pun intended).

Honestly, even up around 100-150k miles, a shimmy at speed could a whole host of things. You may find that it's not just one thing - if parts are all original it could be a combo platter. Especially at 200k+. If it were me, I'd attack it from one of two ways, decided based on the budget. If I had a small budget, I'd start with the cheapest fixes/replacements and work my way up until the problem was solved. OR if you're bank rollin', replace your bj's, steering components, get your socks balanced and aligned, and hope that all that dough you just dropped fixed your problem.

Don't borrow trouble. Start by balancing and rotating your tires, although replacing the lower ball joints on general principle is never a bad idea if you don't know when they were done last.

Great. Will take a look under there tomorrow. Tires about 50% worn.

Rotate tires first. If any change, your issue is probably wheel/tire related. While wheels are in the air, check for any play at 3-9 o'clock axis and 12-6. This will give you a clue as to TRE's condition (3-6). If any play at 12-6 that may be BJ's and you want to get on that fast. Then eyeball your rack bushings as someone turns the wheel back and forth. Any side-to-side movement of the rack means get the Wheeler rack bushing kit ASAP. This made the biggest difference along with wheel balancing in my case.

I had a bad case and it was resolved as I did:

-rack bushings
-shocks
-new tires, road force balanced
-swaybar bushings and endlinks
-alignment.

My TRE's and BJ's were tight. It seemed like each thing I did raised the onset of shimmy by 5 mph or so. Now it can go 95mph (as fast as I dare, and near as fast as I can) and is still rock steady.

As said above, normal wear can create a cocktail of issues over 15 years and 150-200k. I only changed things that needed it. I am going to do BJ's as PM before I hit 200k.

Does the entire truck shake and then go away above 80?

I think I worded my reply wrong. The struts, lbj's and outer tie rod ends fixed the shimmy completely. I'm not sure what one it was that fixed it though. Steering rack is nice and tight as well.