When I've got my bike loaded for touring,(panniers, trailer, etc.) and I go up the supersteep hills, my drivetrain slips a little. The chain is still fully engaged on the cogs and chainrings, but the freewheel hub interface(?) seems kinda mushy. Am I about to spend some cash?

I've got an '87 Le Tour, the only thing updated are the shifters and derailleurs, if it helps. (6-speed freewheel with a double, and I already did a search, mr. smarty pants!)

When's the last time that you had the freewheel off? Spin-on freewheels will sometimes tighten themselves a little on the hub the first time that you crank up a steep hill but that usually only takes once to get it tight. If you have prawls skipping I'm thinking it's probably time for a new freewheel. Unfortunately, I'm thinking that good quality ones are probably getting progressively more difficult to find. Good luck.

Harris Cyclery has Suntour & Shimano freewheels ranging from $20 - $50 US. NOS freewheels can be found aplenty on ebay too.

I'd double check that your not getting chain & cog slippage first. Closely examine the cogs for signs of wear. Have you noticed if it's skipping on more than one cog?

You could also try lubing the freewheel body. My favorite is Phil Wood oil for freewheels.

Thanks for the help guys. I see Nashbars got their own 6-speed freewheel and they've got Shimanos, too. I'm not really worried about finding parts for this older bike, since they're not all that rare YET, and I'm not giving up 27" wheels until I can't find the $10 tires anymore! Besides, I like the way they ride.
I think what's going on is this: I've been taking training rides around the hilliest sections of town fully loaded plus a 50 pound bag of gravel in my trailer. I think the freewheel must be tightening on down since it's never had that much force applied to it before. Like I said, it hasn't slipped, so much as had a "mushy" feeling when I'm on really steep hills. If the chain had slipped on those hills I'd probably have wrecked from all the force I was exerting on the climbs! If my freewheel shatters into a million pieces, I'll let you know! Thanks again for the help.