Vehicle won't crank

1999 Chevy Suburban, 120,000 miles, 5.7 liter engine, 4-wheel drive.

Several weeks ago, I got in the truck and ran some errands. The truck started and ran fine, with no problems. I came home and when I went to restart the truck a couple of hours later, it wouldn't start. The starter was clicking, lights and radio came on, but it would not crank. We replaced the starter and nothing. We replaced the battery, and it started right up. Thought the problem was solved. Two weeks ago, we went to start the truck and, once again, it wouldn't crank. We have lights, radio, etc. You can hear the starter clicking, but it will not turn over. It will not even attempt to start. We replaced the battery again, thinking that maybe something was draining it, but still nothing. So, with a new battery and a new starter, any suggestions? At this point, I can't even get it anywhere to have it fixed unless I have it towed. Can you imagine how much it will cost to tow a Suburban? HELP!