UTG has limped a lot and seems donkish, SB has done nothing to speak of but hasn't given me the impression he is a decent player at this level thus far. I just call the turn, looking back I now see the FD but wouldn't villain lead that on the flop? His range seems like 70/30 weaker pairs/flush draws (some villains don't lead these but almost all would lead a pair), and by raising I fold out all the hands I get value from on the river that bet but also get called by FDs (maybe?).

The donk is behind on the river, does it seem better to just call to trap his money in the pot, as nobody can really have a hand strong enough to call a raise here that doesn't have me beat?

Full Tilt Poker $0.10/$0.25 No Limit Hold'em - 5 players
The Official 2+2 Hand Converter Powered By DeucesCracked.com

UTG: $24.75
CO: $36.50
BTN: $4.50
SB: $25.90
Hero (BB): $28.65

Pre Flop: Hero is BB with 6 4
UTG calls $0.25, 2 folds, SB calls $0.15, Hero checks

Flop: ($0.75) A 4 8 (3 players)
SB checks, Hero checks, UTG checks

Turn: ($0.75) 4 (3 players)
SB bets $0.75, Hero calls $0.75, UTG calls $0.75

River: ($3.00) T (3 players)
SB bets $1.75, Hero?,

I'm not sure I agree with your thoughts here. SB has lots of possible hands are calling a modest raise from you, but if you call here the maximum you'll bet getting is another $1.75, and that's only if utg sticks around.

Raise and fold to a 3bet on river IMO. Around $4-5 is what I'd do here. I think a worse hand can call but not re-raise. SB can definitely have Ax. Donk can have lots of hands. I see what you mean though. I don't think it's as thin as you do. That's prob the difference. Then again from my own experience in raising these rivers it does seem like I get 3bet or called by better a ton--from memory.