I have about the same problem. I was using an ATV but I only used it for backing in my boat and after a couple of carburetor rebuilds and other problems, I looked at several trailer dollies. I decided on
TRAX Power Dolly Systems and am very happy with my choice. They're out of Canada. I talked to the owner before I bought it and it's an interesting story of how he started this company. It cost me plenty but still way cheaper than another 4-wheeler, or some of the more expensive trailer dollies. It took me a few tries to get it figured out just how to maneuver my Ranger Z118C but now I use it all the time and it works great. It has a switch for low/high speed. On high it goes faster than most other trailer dollies. I also have the lip to my garage and I just get it up to speed and it goes right over the lip with no problem. I've had it for about a year and a half and it still works great with next to no maintenance, just charge the batteries.
I highly recommend the TRAX trailer dolly!