There is no true "proportioning" valve on your truck, & stock ones aren't adjustable even on the years that had them. It's actually a "residual pressure" valve, which is also not adjustable.
But the most likely reason is bad REAR brakes. Jack up the back, & pull the wheels & drums. Then look at the autodjuster cables. If they're slack, buy a compelete brake hardware kit from a good parts store (stay OUT of the "zone") & replace everything. Might as well do the shoes since they'll be off anyway. Get lifetime ones. If the drums aren't smooth, you might as well get them turned.
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