The mountain hawk / Charles G. West.

Enhanced descriptions from Syndetics:

Mountain man Trace McCall has seen enough of “civilization" to be content with a simple existence living off the land. He keeps mostly to himself—except for visiting with pretty neighbor Jamie Tresh and occasionally crossing paths with the local Blackfeet tribe.
But forces beyond his control are about to put Trace's peaceful life on the line.

Trouble starts when he decides to help some homesteaders make their way to Fort Bridger. The journey puts Trace on the wrong side of two violent men—and a group of renegade Blackfeet on a murderous mission. Then he finds out Jamie's been abducted—possibly sold into slavery, or worse. Now it's kill or be killed as Trace's pursuit of the kidnappers leads him ever deeper into danger among warring Indian factions and hostile white men in the world he'd hoped to leave behind…