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Some ISP's restrict you to a single hardware address (confusingly called a MAC address - Media Access Control address), so I'd try using the "MAC Address Cloning" feature in the D-Link (I'm assuming it has such a feature, since virtually all such routers do these days) to set it's MAC address to the same address as the Linksys.  You'll probably have to go into the Linksys config or status display to see what it is, but that should be easy.

Otherwise, check that you have the correct type of ethernet cable.  I'd expect you to need a straight-thru cable between the router and the cable modem, but if you have a crossover cable that could break it.  Some devices have a feature on some ethernet ports that auto-detect the polarity and will work with either a straight-thru or a crossover, which could be why one works and the other doesn't (though I don't remember the Linksys having that feature).

HTH - Good luck!

try resting your cable modem with everything hooked up, also try leaving it connected for a while, sometimes the routing tables on these broadband connections can get congested and if your ISP doesn't reset their routers every now and then  the DHCP might be having a hard time finding you an IP address...  although if you cloned your MAC address you should have instantly gotten the one you previously had leased.