trans shift situation

[BIG CHEVY]

ok........79 k10.....350/350/203.           my transmission has been the most solid thing this truck has had goin for it. never even once did anything to make me feel like it wasnt brand new. shifts perfect. i mud it a little and am pretty easy on it and i clean it well. heres the problem. mudded last time end of november.  drove it home the next day and all was still perfect. p-washed it but didnt clean it like i usually do. went out of town for cpl weeks and finally wnet to fetch it today. crunk great....warmed up great....swept out the bed.....eased on out. wouldnt come out of first. this is bad. drove about 1.5 miles (slowly) 2000 or less on the tach.......checked fluid. was great. please help.....what is wrong? trannies scare me to death. and lawyers. are there any ways i can check to see if the prob is serious or small? what are the simple things? 4 days before christmas is not a good time for this. sorry for the length but i always felt all details help ppl make the best choices....thanks guys......ill be waitin

[BIG CHEVY]

extra details..........started off once way down in first and then shifted to second and felt it do it manually but didnt hit D or OD with the manual shift. also when i come off the gas  to slow down it drops to idle like i think it should. ( doesnt keep high rev like in a manual left in first)

[thirsty]

Check your modulator valve vacuum line. That is the easiest and cheapest thing I can think of. Hope it is an easy fix for you right before the holiday.

[BIG CHEVY]

Someone have more? When i go i wanna go with options. As many as i can get.

[bake74]

     As long as you cleaned it good like you said, and checked fluid, made sure linkage was not stuck, Thirsty has the right idea, though I am sure more will chime in, but you have to give them time to respond.