The commercial you're talking about is for Levi's 501 jeans. It's in black and white and shows two teen girls running through the woods and they stop to spy on a muscular, shirtless guy bathing in a quarry-type lake. The girls find a pair of jeans they think belongs to the guy just as he's about to stand up out of the water. Expecting to see his naked body, they're puzzled when he steps out wearing pants. Then, they look over down at the other side of the lake to see an older, balding, bearded mountain man kind of guy. He's wading in the water, trying to find his pants.
It's a pretty funny commercial that I haven't seen in probably a decade, but I remember it quite well, mostly due to the music. I've also been looking for a long time. In searching on this site, I came across your post and I started to remember the music as if I'd heard it just yesterday. Remembering some lyrics, I went to getlyrical.com and did a search. Finally, after all these years, I have the answer:
Artist: Stiltskin
Song: Inside
Album: The Mind's Eye (1994)
The band is from London, so if you live in the UK, it shouldn't be too difficult to find that album. In fact, I'm sure it's easier to just order it from amazon.com, where the track can be purchased as a single. Best of luck. I hope this was helpful!
-John