This tech-stock rally could be due for a detour. Shares of Garmin (GRMN 0.15%) have been a big winner this year, up more than 30%. The global-technology leader, recognized for its navigation and communications devices, has benefited from an ongoing diversification of its product lineup. Indeed, the company's latest-quarterly update was highlighted by solid growth and record profitability. The headlines are positive, but is that enough to keep the rally going? Here's what you need to know before buying Garmi ...