When is the last time you had an alignment? If it's a 4X4, 1984D250 posted some good ideas to look at. To add to that, look at your front drive shaft and the U-joint going in to the front diff. Speaking of the front diff, have you had it serviced in a while? Then check your output or CV (constant velocity) shafts to your wheels. If it's a 2WD, then bearings, hubs, break rotors or calipers could be the culprit. Look and tweak one thing at a time, and start with the easiest, least expensive items first and then move on to the more complicated and costly things. Keep us posted on any progress and don't be afraid to ask for some more help/advice.
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