We made this journey September 2016 and wanted to share.
Everyone told us (both internet and face to face) that this journey could not be done by local bus and that we would have to get a tourist shuttle via Guatemala City at a cost of $52+ per person and would take approx 10hours.
In Xela we took a taxi from our hostel to the bus station (Terminal Minerva) costing 40Q and taking roughly 10mins. Once at the terminal we asked for the bus to Santa Cruz Del Quiche and we were led to the right bus.
Bus No 1 from Xela to Santa Cruz Del Quiche left at 8am and cost us 25Q each. We got into SCDQ at 10:30am.
Bus No 2 (minibus) from SCDQ to Uspantan left at 10:50am and cost us 30Q each. We arrived in Uspantan at 1:30pm.
Bus No 3 (minibus) from Uspantan to Coban left at 2:10pm and cost us 30Q each. We arrived in Coban at 5:00pm after driving over some bad roads and through a storm.
We stayed in Coban overnight to break up the journey.
The next day we had a leisurely start and after breakfast got a bus from Coban to Lanquin which left at 10:00am and cost us 25Q each. This got into Lanquin at 12:00pm.
We could have made the whole journey in one day but had spent enough time on busses the first day, plus time was not an issue.
If anyone wants any more info please let me know.