There is no valve on the manual transmissions. But definitely make sure you check oil levels. Doesn\'t hurt to drain the transmission and get the fluid replaced, generally speaking, you want to do a drain and refill every 15K-30K miles at most, to keep the transmission in good shape. Depending on the driving conditions, draining and refilling it every or every other engine oil change interval is considered good practice.
The issue that the previous poster alluded to doesn\'t sound like what you have. Those cases, 2nd gear tended to keep popping out of gear or was hard to engage.
Your case, sounds more like the truck is low on power when you shift into 2nd gear. Assuming this is the 5-speed transmission? What engine is in the truck - 4 cylinder or 6-cylinder? Most of the standard shift Tacomas that I\'ve driven have a really short 1st gear, and a relatively long 2nd gear.
Could be a tune-up issue - when was the last time you have a comprehensive tune-up? Plugs, cleaned MAF sensor, replace PCV, new air filter, fluid exchange, etc.? Could also be a glazed or worn clutch disk?
Does it seem to get better or worse under any conditions? Most noticeable when driven after being parked over night? or after it has been driven for some time?