Steering on a 1978 ford f-150?

Could be a number of things that could cause this.
1. Check & adjust your tyre pressures if required.
2.Are the tyres wearing evenly on front & back?
If they are wearing on the edges then alignment is out between that axle set the tyres are on. If its the front check the tracking, (if you have to take it to a garage for this make sure to have all wheels checked).if its the back set, may be suspension bushes.
3.Get someone to follow you down the road, is it crabbing? Rear axle out of alignment (Suspension mounting & bushes worn or even chassis problem).
4.Lift front wheel off the ground, grab wheel and push & pull 1st in the steering axis to check track rod ends, then on the vertical axis to check the upper/lower hub/suspension ball (?) Joints for clucking & visible movement (try this last one with the wheel on the ground as we are trying to test with the joints unloaded).
5.Have a good look at all the rubber protection boots while there & clean/ grease if any points that you find.
6.If you can lock the steering wheel, try whilst the wheels are up testing the steeringrack or box for play or noises.
7.Check your service manual first, but track rod lengths should be the same.
8.Anti roll bar linkages & mounts should be free from play too.
9.Power steering ok? Check fluid levels ect. The drive belt usually squeals at extremities when they are dying!).

These are the things I would do first & applicable to most cars & trucks.
Hope this helps or brings up something you have not yet checked. Sorry if I could not point too a particular fault that might be typical of your trucks model.
Good luck, hope its easy & inexpensive!