The way I read "occurs at 60-65mph and the wheel just shimmies right and left until i stop" is that it rides fine up until that point. Do you remember if there is a bump or turn or something that happens right before the shimmy starts(pushes it over the edge so the speak)... i.e. something to get that started? . I was going to ask you to measure your height to see how much sag your have from stock springs but that would just mean more caster and i think that wouldn't cause the shimmy. bad shocks or broken rubber bushings can cause some crazy things i think you can almost visually see issues there. Not to contradict those that know more on the subject above, but my process would be to check the most dangerous first and work your way in.. the studs that have been mentioned have to be checked (i am in need to check mine again as i recently did the BIRF job), those can literally kill you and others so I would check those, and the nuts closest to the shield for the brakes may need a socket that is swiveled for you to get to them and check.
Then if those are good, I would disconnect the sway bar and take two jacks and put one on the frame and one on the opposite side but axle and start flexing out both the radius arm bushings (mostly the ones at the frame) and the panhard bar bushings so you can look for cracks and worst to see if any have split and give way to to a geometry change at speed. The geometry change which would allow the wheels to be out of control or out of parallel. do each side with opposing jacks (if that makes sense). While each step ends up with one wheel in the air, check wobble of the lifted wheel at 12 and 6 o'clock and 3 and 9 o'clock. play at 12 and 16 can be bearings loose wheel bearings or trunnion bearings (mind were broken i found a few weeks ago, didn't know it but after cleaning old parts found out: PM for the win here). 3 and 9 o'clock can be wheel bearings too, but is often a sign of tierod ends. might even be something in the steering box, (not sure that is going to be the issue).
really get up close and look during the flexing to see if you have any cracks or breakages of all those rubber bushings.
Just a thought.. you said wheel bearings were changed recently and i assume trunnion bearings were part of that service, there is a shim that is used on those, you might ask the person who did it, if they checked preload and used the shim or threw it away. Mine seemed good with the shim so i didn't mess with it that much, but it is something to think about.
Also on an old Ford Courier i had the same situation you have.... different vehicle... but fixed it once i realized my father hadn't tightened the lugnuts. it was 55 not 60 where its started to make us have to change our underwear...
I really don't think the issue you have is going to be expensive nor is the cost going to be anywhere near the cost of finding another vehicle so i wouldn't be too worried about that.
hopefully this helps and you find it and can post it back what solved it. someone else running original and stock height will likely need this info someday.
This will probably be my last post as the truck is pending sale and I've moved on to a Ford Superduty. I just came to the harsh realization that my 80-series can't practically be a daily driver and tow vehicle without significant time investments.
The wobble has always started while driving on the highway. The highways in question are the 215 in Vegas and the I-15 to Salt Lake City. They are good roads, but like most highways, they have some construction areas that *may* have triggered the wobble.
The trunnion and wheel bearings were replaced at the same time, along with the seals--the full kit was purchased and installed in Nov '19 from Cruiser Outfitters. However, the first wobble did not happen until April '20--should I have expected the bearings to come loose after a certain time? Are they like lug nuts where you should recheck them after a few miles?
Speaking of the lug nuts, they were tightened to 108 ft/lbs because Costco. I should have paid attention to this as many have pointed out on this site (I did search this!) that the lug nuts for aluminum alloy wheels on a '94 should be tightened to 76 ft/lbs.
TREs feel tight and I cannot appreciate detectable slop or play when turning the steering wheel side to side when the truck is on the ground. All of the dampers, springs, and TREs are original to the truck and they probably should be replaced, but honestly, I tend to dying covid patients at work and have neither the time, nor energy, to tend to an aging truck at home--I'm pretty smoked after work.
I checked the stud nuts and they were loose. Tightened those to 75 ft/lbs (I read the range was 71-80 ft/lbs for stock). The studs themselves where tight. The front sway bar bushings were shot. These are the most likely culprits, and ironically, I would have found these issues without posting on the forums. I was just thinking about how things would have turned out without the drama--without the sheppards pouncing on wounded sheep--perhaps my love for the 80 would still be there, but instead, I'm left feeling bitter and abandoned by the very resource that had helped me so much in the past. As a result, my 80 was on the receiving end of my frustration--hence the sale.
Anyways, I hope someone, somewhere, will find this info useful. As I sign off, all I ask is that you respect one another and be patient with the newbies. This, for the most part, is a great community and resource. Please understand that some of us aren't the most mechanically inclined, and don't have hours to spend searching forums and watching videos. A quick shout out to Kernal, who has always been kind and patient with me--even taking the time and effort to reach out to me and point me in the right direction. I wish you all good health and thanks.
