Power trim/tilt question

My boat is not unwrapped for the spring yet, since we still have snow on the ground here in Vermont. But there is no ice on the lake, so it's about time to start thinking about what needs to be done this year. One problem I encountered with my boat last year, had to do with the power trim. I have a 1990 Wellcraft Sport 20 with a 1994 150hp Mercury Black Max ob. I have only one switch (up/down) for power tilt/trim on the console, and that switch is labeled "power trim". While I know for certain this switch works for tilt, the trim does not work at all. I know some boats have independent trim and tilt switches, so I wondered if maybe it was only for tilt, but a mechanic told me it should work for trim also. #1 - Is this true? The motor tilts just fine when the boat is stationary or moving at a low speed. When I try to trim it however at a higher speed, I get nothing. The mechanic told me that it could be something as simple as a $35 part or I could need a whole new power trim unit which is of course big $$$$$! He said they would have to "get into it" to look at it. I need advice or expertise on how these things work, and what I could look for before I bring it in. Any ideas? I appreciate all the great advice I read on here.

Thanks,
Chris