Note that if this is an internal routing problem, you may want to try the following: externally feed your 50 MHz clock to two FPGA inputs instead of just one, both being GCLK pads. Then you'll use one as your 50 MHz clock, and the other as the input for a DCM. You can try that virtually of course - just modify your RTL and your UCF file (to have 2 clock inputs instead of 1), and see if it solves your problem. If so, you'll need of course to physically route your clock input to two pads instead of one accordingly.
Note again that if you use this clocking scheme, you'll need to handle sync between the two clock domains.