You said the noise was coming from the right rear...correct? If it were me, I'd pull the old tensioner out and see if the spring inside seems weak when you push down on the plunger with your thumb. It's easy enough to remove by first pulling the wheel and then accessing it through the wheel opening (behind the rear portion of the dust shield). Need a 27mm socket (upper tensioner needs a 27mm DEEP socket).
I ran down to the local U PUll this afternoon to get a first hand glimpse into how the upper tensioner works. They had just gotten in a '98 SOHC w/223K miles. I pulled the intakes, thermostat housing and LH valve cover to have a look (don't need to remove all this to access the tensioner...I just wanted to have a look at the timing chain/cassettes inside). The tensioner still had plenty of resistance to keep the cassette/guide firmly against the chain. Who knows if it had ever been replaced. As a reference for resistance, I held it in my hand after removal and pushed down on the plunger with my thumb. I can't quite compress it all the way in this manner...and, I'm no 98lb weakling. I don't have a new one to compare, but seems plenty strong to me to hold the plastic cassette in place on cold start.
The conclusion I've reached is that odds are good that failure can be postponed by routinely replacing the tensioners; preferably before the dreaded "rattle" is heard. However, if the plastic cassettes are prone to breaking simply due to becoming brittle with age, the only true solution is to replace the plastic cassettes/guides along with the tensioner. That probably isn't a terribly difficult job to do on the front. But as I understand it, the rear one requires engine removal...and, the expense of paying someone to do that can't be justified for most owners of these aging trucks.
