Rob McGrail

HI People,
I have a 2004 GMC Savanna 3500 extended cab with a 6.0L and the 4L80E transmission. You typically see them with a square box and 4 wheels on back. For over a year I never went highway speeds. wife was in Hospital 2hrs away for 2 weeks noticed i was running warm 1 night than next day it will go into gear when started but when it reaches operating temp it plays stuck in neutral in every gear. I did experience shuddering around 45 same night coming home. I hit 45 and wheel shook so bad if i didn't brake i would have lost control. After a lot of reading and being W/O a vehicle. ( I live in Mass and with no heated garage I've been forced to wait for spring)

My question is before buying a used transmission cuz I cant swing 1500 to 2k for a rebuilt, Could it be
just the torque converter? should I try replacing that first for $200 or should I keep trying to find a used one for outrageous money. or an unemployed or ambitious transmission tech who wants to make $500 and I buy the parts. cuz I cant afford a shop. I'd say i'm on a limited income but to say i'm looking up n I mean way up at broke would be more accurate. I've even considered trying myself with a 4l80E rebuild book for dummies I found but I wouldn't think anyone without prior internal trans work shouldn't think twice about it because it's nothing like working on a motor. I know that much!
If anyone has any ideas or advice it would be greatly appreciated cuz I grew up in Boston now live in Berkshires and it takes 30 minutes to friggin drive anywhere never mind fkn walk. Help Please if you even think you have input or advice??!!
I know motors It was my major in high school but trans are a whole different ball game. and If I spend 500 on parts and screw it up i'll wanna kick the jack out from under it while im still there. The van is actually too valuable to sell as is and I don't want to buy someone else's headache for whatever I can afford.
Thank You
Rob McGrail
978 504 9998