A buddy of mine said it could be due to bad teams mounts jerking the trans when I disengage the clutch plate, but the shifter doesn't seem to jerk so I'm wanting to rule that out.
Another reason the bad mount theory doesn't sound right is because all of the rotating elements that comprise the drivetrain are always spinning in the same direction (when moving forward anyway). So, it would seem to follow that; if the engine/transmission were not securely mounted, the release of the drive load would consistently torque the drivetrain in only one direction. And application of the drive load would create torque in the opposite direction. Your steering system, on the other hand, is pulling unpredictably/randomly to the L or R when the driven load is uncoupled from the driving force.
but when I push in on the clutch, usually when shifting from 2nd to 3rd my 87 k10 with a 4 in lift, no leveling, will jerk to one side or the other
Maybe the erratic steering issue is completely unrelated to the operation of the clutch and transmission - it might just appear that way because it happens when you shift gears.
Consider this:
Assuming you have power steering - and that when you disengage the clutch, you also release the accelerator. When you do that, the engine RPM's fall off and - to a certain degree - so does the performance of the various belt driven auxiliaries. Think things like - alternator output voltage, A/C compressor discharge pressure and pressure/flow rate of the hydraulic fluid that the PS pump uses to stroke the steering gear.
Next - and this is just a possibility - consider that you might have a worn/binding component(s) in the mechanical linkage of the steering system. It could be anything; a really trashed tie rod end or ball joint, excessive play at the pitman arm to steering gear output shaft, worn connecting link ends, steering gear loose on the frame, loose front wheel bearings, even excessive lost motion in the steering column. Worn steering gear/ PS pump internals. Air in the hydraulic system or low fluid level.
Anyway, what if the steering system is only able to keep this imaginary condition in check when the engine speed is up (and the pump is delivering fluid to the steering gear at a high pressure/flow rate). Then, when you go to shift gears and let off the gas, the pump output drops off and the steering system is left momentarily underpowered. Maybe the linkage shifts out of alignment, maybe the pitman arm slips on the pitman shaft, one of the loose connecting link ends begins to wobble...you get the idea.
It's kind of like the loss of braking power/control that occurs when the engine stalls and the vacuum supply to the booster is lost.
It has taken a long time to get to the point, but what I am suggesting is that you get under the truck and take a good look around. See if you can find any obviously worn parts, make sure everything is tight and the hydraulic system is clean, full and purged of any entrained air. Check your wheel bearings, rag joint, and connecting link ends. Stroke the steering and watch the driven parts - all the way from the pitman shaft right down to the tie rod ends. Look for any jumping or lost motion. I hate shotgun repairs more than anyone. But in this case - if a thorough inspection doesn't reveal anything obvious - you might want to spring for a new PS pump.
I don't know anything about lifted trucks but others here do - know a lot - and maybe they have suggestion that is more helpful.
BTW - I can't recall any of the SB's I have driven as having loose steering systems. If they did, it wasn't for long.
Good luck. Come back once in a while - and not just when you're in trouble. Many of us here are on our last legs - we could use some young blood.
