coloradoan - it might seem you have received confusing answers but both posters are giving you relevant information.
First, my recommendation would be to stay away from diplexing completely. There are so many problems you might run into and they will be very difficult to diagnose and fix.
Second, the technical stuff. As you can see from the posts there are a number of signals in the SWM system and in WHDVR that the sat cables carry, and if the diplexers you use don't support those signals correctly things won't work properly. Also, if the signals from the cable internet system overlap any of the DirecTV signals it won't work either.
Jerseymatt's comment about diplexers and the SWM control signals are important but not 100% correct. It is true that many diplexers won't pass the SWM control signals correctly, but not all. There are a few diplexers that DO pass the control signal, even though the spec indicates they might not. In fact, DirecTV's original stand-alone SWM multiswitches had built-in diplexing for off-air signals, and for a few years I had a perfectly good diplexing system for off-air signals using SWM. However, I had to abandon that when I installed the WHDVR system because the off-air signals and the WHDVR signals (from the DECA devices) overlapped.
The next problem is the signals that are carried on the cable internet. Most cable internet cables carry three different sets of signals. There are the signals from the internet and the signals between your cable modem and the cable company. different cable systems use different frequency bands, for example your signals from the internet might be at around 750Mhz and the control signals back from the modem might be at, say, 54Mhz, but mine might use different frequencies. You may also have cable TV signals on the cable, because even though you don't subscribe to cable TV, that does not mean the cable company has filtered them out. Without knowing what frequencies your cable modem uses for both the upstream and downstream paths, it's impossible to draw up a frequency usage "map" for your particular system, so it's difficult to predict whether or not you could set up a diplexing system that will work. SWM, WHDVR, cable modems and diplexers don't play well together.
Hence my recommendation - don't do it!