Vibration in drive train

We have a 2000 land rover discovery, about 90,000 miles on it. We bought it about a month or two ago now and haven't really had any problems with it. There appears to be a small leak with the ACE fluid, but its a very slow leak. Over the thanksgiving weekend we drove it out of town and back about 300 miles each way and it drove wonderfully. The day after we got back we went out and ended up on a dirt road that had quite a few pot holes and bumps in it that rocked us a bit. Once we got off that road traffic was horrible and we crawled along for an hour at 0-15 miles and hour. When we finally got out of traffic and could speed up we noticed a vibration/shaking as we accelerated to around 30-40 mph. If we let off the gas it cruised smoothly or if we accelerated faster than 40 it smoothed out. We thought maybe the tired were off balance and had them checked and balanced the next day, however it is still vibrating/shaking. Any ideas or input would be greatly appreciated. Thank you.