my truck seems to swim around at 55

Is this a new behavior?

Did it coincide with the installation of the 33" tires? It's entirely possible the issue is with the tire. If you have very good dry pavement and big gaps in the tread like an ag tire then you have very good traction pulling you all over the place.

Also, there are good big mud tires and really poor big mud tires (which cost as much). I've seen untrue 35 inch tires start bouncing the front end of a truck at 55 MPH. Looked funny, but obviously unsafe.

theyre a mud tire but theyre starting to get wore i think every other outside lug is slightly more wore down than the other and it was good dry pavement when i noticed it

i havent had a chance to drive it with regular tires because i just got my licence and its been sitting for 2 yrs due to tranny failure and ive spent the last year working on it (its a 1985 gmc half ton 4x4) and i bought the tires and rims for it just before i started driving it

i also cant put the ones that were on it because the deal with my mom was if she took me to get the tires (got them off craigslist) then she could have the regular tires that were on it because they were still good, and the same size that she needed for her jeep

and i have the pressure at 40 psi in them right now and theyre rated for 35 psi cold

The bigger tires, and a bad drag link will cause that. Before you can realy diag the problem you need to fix the known bad parts. Also do you have a steering stabliser on the front? If so, it may be bad, or you may find you need one.

Now a 85.... that the old style right??? They used to have a issue with braking the frame rail where the steering box bolts on. Check that aera real good for cracks.

I belive your issue is in the front, not the back.