Sounds like death wobble. Check all of your front suspension and steering. Look for loose/worn tie rod ends, ball joints, hub bearings, etc. Pay close attention to your trackbar ends, make sure they are tight and not worn. Make sure all the nuts/bolts are tight. If all that checks out go for an alignment and tire balance. That should cover what may be causing your issue. You can also do a search here and find numerous threads about death wobble and how to fix it.
Here are the results of one search.
Don't run out and buy a steering stabilizer thinking that will fix it. If your front suspension/alignment is good you should not have DW without it.
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