1999 Chevy Silverado Delay Engaging from Stop

Over the past weekend I noted that after about a 2 hour drive at highway speeds (in 2WD - automatic transmission) when I parked and then about 5 minutes later re-started the truck and placed it in Reverse and stepped on the gas, there was about a 1 to 2 second delay where I heard a scraping, jangling sound and then the transmission engaged, hard, and then proceeded to operate in Reverse as normal. This occurred a second time as well after stopping on the way home. I then noted that when I stopped at a traffic light (in Drive), when the light turned green I got the same behavior, there was a jangling, scraping sound and then the transmission slammed into gear. This has happened a couple of times over the past few days, and once while again stopped at a traffic light and I was in 4WD (snowy day). The transmission seems to behave most of the time and shifts up and down while in motion just fine. Any ideas? Is this something serious? Can my mechanic diagnose this or do I need to take the truck to a specialty transmission shop? Being a possible transmission issue, I know the nightmare it can be to get a true diagnosis of the problem that won't cost me thousands of dollars just so a shady transmission shop can make some big money quick. Thanks guys!