Take a look at your undercarraige, and hose it down. Caked up mud on a tire or rim could be enough to throw it out of balance and cause a vibration.
If you hit your rear real hard, would it have been possible that you also hit the driveshaft a bit? They can bend, or get dented and can cause a vibration.
Now for a BroncoBill78 story. In 1999 I had a brand spankin new A class motor home that I took cross country. In Oklahoma I took a turn too tight, and caught my tailpipe on an upright, forcing it under my wheel which caused the muffler to twist and hit my driveshaft. It got bent, and at 10 MPH there was a noticible vibration. I couldn't go faster than 30 for fear of ripping the rig apart. I was only about 2,000 miles from home.
I called FORD and parts stores, because the unit was so new, there were NO driveshafts available any where in the country! I asked them to pull one off of the assembly line, and they wouldn't do it.
Thank God, I stopped into a Chevy dealer and went into their parts department, and asked if there was anywhere that I could get my driveshaft balanced?
The gal behind the counter knew of some guys in Enid, OK who built driveshafts. SO I drove the 50 miles to get there, (it only took two hours) and in the morning they pulled my drive shaft, cut the ends off, welded a new pipe in between, spun it on a lathe, balanced it and reinstalled it, all for less than $200. I thought for sure I was going to get *****! An out of stater, obviously up the creek without a paddle, and they treated me as good or better than a local. Going accross the country, I think that the people of Oklahoma are amoung the nicest we met.
IF it is a wheel out of balance, you might have a friend drive behind you at fify or sixty and look at your tires, he might be able to see them wiggling.
