I've flown over Thunder Ridge a couple of times in an airplane, and I can confirm that the brush looks pretty low -- at least from the air. It is a private field though, and really only should be considered as an emergency choice. It is generally easy (and advisable) to stay high enough in that vicinity to keep either Heber or Duchesne within gliding range. The CUP comments (generated by my website at http://www.soaringdata.info) for that airport say:

PRI (UT83)|RY 13/31:4000x48-ASPH|WILDLIFE ON RY, RY 31 NOT RECOMMENDED FOR DEP, RY 13 NOT RECOMMENDED FOR LND DUE TO RISING TRRN.|OWNER DESIRES CHARTING.|

The comments also give an owner contact and phone number.

Lynn Alley
"2KA"