All the roads out on The Navajo are bad. They are narrow and the asphalt is old and in bad condition. It's like a third world country out there. I have been traveling those roads for the past forty years. In June I pulled our fifth wheel to Colorado and back across The Navajo Reservation. The section of 191 between 160 and Bluff is one of the worst roads I have been on in a while. There is one section about five miles long where the asphalt has deteriorated so bad it feels like it going to rip the tires off your rims. Of course that's not going to happen you just need to drive about 50 mph and take it easy. I haven't been on the the section of 191 south of 160 in many years, but I that section is more traveled and should be better.
Regarding 491 North of Gallup, that too is a major route and should be better than most roads out there. On the other hand 64 west of Shiprock towards Monument Valley is terrible. I recommend taking the most direct route to Bluff from Gallup. 491 Northbound to 264 Westbound through Window Rock. Catch 191 Northbound all the way to Bluff. Be extra cautious out there it's open range and the locals enter and leave the roadway without notice.
2018 Arctic Fox 27-5L
2018 RAM 2500 6.7L