Is it easy to find a taxi for a long route such as Escárcega to
Xpujil? I would like to have an idea of the cost.
Colectivos can get you almost anywhere, and cheaply - but it may involve a bit of a wait, some inconvenience and minimal comfort. Some Spanish skills and creativity in communications (and a positive outlook) will help. No idea what the cost will be but it'll be cheap.
Colectivos might get complicated because of my luggage.
While many colectivo drivers will be very accommodating, some may not, and you never know. I'd be very careful about how much you are bringing along, and would hope you will be lugging no more than a medium-sized backpack. That may have to go up on the roof of a full collectivo. If you've got much more than that, it's going to greatly complicate things for you (not just the hassles of carrying a lot of gear, or trying to keep it with you inside a collectivo where it might displace another paying passenger), but if you've got more than a single, wearable pack, going solo you will need to leave unattended it as you tend to critical tasks, and the risk of theft should not be minimized).
The road between Escárcega and Xpujil is a major highway - an understatement. It doesn't look like a US Interstate or the Trans-Canada, but it carries a LOT of traffic, and LOTS of trucks, even at night. It is the primary road across the Yucatan Peninsula. At night traffic certainly drops off, and there will be times/places where it's quiet and you don't want to be stuck after dark, but the trucks will be rolling though all night long.
Palenque is very much on the gringo/tourist trail, and there will be no shortage of international visitors. Between Palenque and Xpujil, things "thin out" quite a bit. Mostly just small towns and tiny settlements, though Escárcega is an actual city with all the services most people would need. It's not a major tourism machine though, like the towns along the coast. Xpujil is a small town.
The road to Calakmul cuts south from highway 186 well before Xpujil. There's a tiny settlement at La Selva, just before the turnoff to Calakmul - that's where your bus/taxi/colectivo will need to drop you. There are a couple of tourist services there and nearby. Try Hotel Puerta Calakmul, a few hundred meters off the highway and technically inside the archological zone; it's not cheap but logistically makes a lot of sense - there's just not many choices around there (it's in Mexico's "empty quarter"). It's a very comfortable, safe place to stay, and whether you stay there or not they can probably point you to a tour operator or other way to get from the road in to the ruins (it's a long, thin, empty 1- to 2-lane road through dense jungle...enjoy the wild ocellated turkeys you will see wandering the road...). Don't skip Calakmul, it's worth the time/effort to get there. There are dozens of worthwhile ancient sites scattered around this area, if you have even a passing interest in ruins, it would be a shame to just blow past them all.